Semiconductor device and electronic device

ABSTRACT

An object is to improve the drive capability of a semiconductor device. The semiconductor device includes a first transistor and a second transistor. A first terminal of the first transistor is electrically connected to a first wiring. A second terminal of the first transistor is electrically connected to a second wiring. A gate of the second transistor is electrically connected to a third wiring. A first terminal of the second transistor is electrically connected to the third wiring. A second terminal of the second transistor is electrically connected to a gate of the first transistor. A channel region is formed using an oxide semiconductor layer in each of the first transistor and the second transistor. The off-state current of each of the first transistor and the second transistor per channel width of 1 μm is 1 aA or less.

CROSS REFERENCES TO RELATED APPLICATIONS

The present invention is a Continuation application of application Ser.No. 12/960,659, filed on Dec. 6, 2010, now U.S. Pat. No. 8,415,665, theentire contents of which being incorporated herein by reference.

TECHNICAL FIELD

A technical field of the invention disclosed herein relates to asemiconductor device, a display device, a liquid crystal display device,and a method for driving these devices.

BACKGROUND ART

Semiconductor devices in which the conductivity of all the transistorsis either n-type or p-type have been developed. In particular,development of semiconductor devices that are constituted only byn-channel transistors has been advanced (e.g., Patent Documents 1 to 4).

Such a semiconductor device includes, for example, a first transistorhaving one of a source and a drain connected to a power supply line andthe other of the source and the drain connected to an output, and one ora plurality of second transistors connected between a gate of the firsttransistor and each wiring.

In order to make the amplitude voltage of an output signal from thesemiconductor device equal to a power supply voltage, the potential ofthe gate of the first transistor is made higher (or lower) than thepower supply voltage by capacitive coupling in many cases. In order torealize this, the gate of the first transistor needs to be in a floatingstate. For that reason, it is necessary to turn off the secondtransistor (or all the plurality of second transistors) connected to thegate of the first transistor.

Reference

-   Patent Document 1: Japanese Published Patent Application No.    2002-328643-   Patent Document 2: Japanese Published Patent Application No.    2003-179479-   Patent Document 3: Japanese Published Patent Application No.    2004-064528-   Patent Document 4: Japanese Published Patent Application No.    2003-222256

DISCLOSURE OF INVENTION

However, in a conventional semiconductor device, even if the secondtransistor is off, electric charge held by the gate of the firsttransistor is lost over time because of the off-state current of thesecond transistor. Therefore, the drive capability of the semiconductordevice is impaired.

In view of the foregoing problem, an object of one embodiment of thepresent invention is to realize higher performance. An object of oneembodiment of the present invention is to improve the drive capabilityof a semiconductor device.

According to one embodiment of the present invention, a semiconductordevice includes a first transistor and a second transistor. A firstterminal of the first transistor is electrically connected to a firstwiring. A second terminal of the first transistor is electricallyconnected to a second wiring. A gate of the second transistor iselectrically connected to a third wiring. A first terminal of the secondtransistor is electrically connected to the third wiring. A secondterminal of the second transistor is electrically connected to a gate ofthe first transistor. A channel region is formed using an oxidesemiconductor layer in each of the first transistor and the secondtransistor. The first transistor and the second transistor have anoff-state current of 1 aA/μm or less.

According to another embodiment of the present invention, asemiconductor device includes a first transistor, a second transistor, athird transistor, and a fourth transistor. A first terminal of the firsttransistor is electrically connected to a first wiring. A secondterminal of the first transistor is electrically connected to a secondwiring. A gate of the second transistor is electrically connected to athird wiring. A first terminal of the second transistor is electricallyconnected to the third wiring. A second terminal of the secondtransistor is electrically connected to a gate of the first transistor.A gate of the third transistor is electrically connected to a fourthwiring. A first terminal of the third transistor is electricallyconnected to a fifth wiring. A second terminal of the third transistoris electrically connected to the second wiring. A gate of the fourthtransistor is electrically connected to the fourth wiring. A firstterminal of the fourth transistor is electrically connected to the fifthwiring. A second terminal of the fourth transistor is electricallyconnected to the gate of the first transistor. A channel region isformed using an oxide semiconductor layer in each of the first to fourthtransistors. The first to fourth transistors have an off-state currentof 1 aA/μm or less.

According to another embodiment of the present invention, asemiconductor device includes a first transistor and a secondtransistor. A first terminal of the first transistor is electricallyconnected to a first wiring. A second terminal of the first transistoris electrically connected to a second wiring. A gate of the secondtransistor is electrically connected to the first wiring. A firstterminal of the second transistor is electrically connected to the firstwiring. A second terminal of the second transistor is electricallyconnected to a gate of the first transistor. A channel region is formedusing an oxide semiconductor layer in each of the first transistor andthe second transistor. The first transistor and the second transistorhave an off-state current of 1 aA/μm or less.

According to another embodiment of the present invention, asemiconductor device includes a first transistor, a second transistor, athird transistor, and a fourth transistor. A first terminal of the firsttransistor is electrically connected to a first wiring. A secondterminal of the first transistor is electrically connected to a secondwiring. A gate of the second transistor is electrically connected to thefirst wiring. A first terminal of the second transistor is electricallyconnected to the first wiring. A second terminal of the secondtransistor is electrically connected to a gate of the first transistor.A gate of the third transistor is electrically connected to a thirdwiring. A first terminal of the third transistor is electricallyconnected to a fourth wiring. A second terminal of the third transistoris electrically connected to the second wiring. A gate of the fourthtransistor is electrically connected to the third wiring. A firstterminal of the fourth transistor is electrically connected to thefourth wiring. A second terminal of the fourth transistor iselectrically connected to the gate of the first transistor. A channelregion is formed using an oxide semiconductor layer in each of the firstto fourth transistors. The first to fourth transistors have an off-statecurrent of 1 aA/μm or less.

According to another embodiment of the present invention, asemiconductor device includes a first transistor, a second transistor, Nthird transistors (N is a natural number), and N fourth transistors. Afirst terminal of the first transistor is electrically connected to afirst wiring. A second terminal of the first transistor is electricallyconnected to a second wiring. A gate of the second transistor iselectrically connected to the first wiring. A first terminal of thesecond transistor is electrically connected to the first wiring. Asecond terminal of the second transistor is electrically connected to agate of the first transistor. Gates of the N third transistor areelectrically connected to respective N third wirings. First terminals ofthe N third transistors are electrically connected to a fourth wiring.Second terminals of the N third transistors are electrically connectedto the second wiring. Gates of the N fourth transistors are electricallyconnected to the respective N third wirings. First terminals of the Nfourth transistors are electrically connected to the fourth wiring.Second terminals of the N fourth transistors are electrically connectedto the gate of the first transistor. A channel region is formed using anoxide semiconductor layer in each of the first transistor, the secondtransistor, the N third transistors, and the N fourth transistors. Thefirst transistor, the second transistor, the N third transistors, andthe N fourth transistors have an off-state current of 1 aA/μm or less.

In any of the above-described semiconductor devices, the oxidesemiconductor preferably includes a non-single-crystal region.Alternatively, in any of the above-described semiconductor devices,oxide semiconductor preferably includes a non-single-crystal regionwhich has a c-axis alignment in a direction perpendicular to a surfaceof the oxide semiconductor.

One embodiment of the present invention is an electronic device thatincludes any of the above semiconductor devices and an operation switch.

For example, in this specification and the like, when it is explicitlydescribed that X and Y are connected, the case where X and Y areelectrically connected, the case where X and Y are functionallyconnected, and the case where X and Y are directly connected areincluded therein. Here, each of X and Y denotes an object (e.g., adevice, an element, a circuit, a wiring, an electrode, a terminal, aconductive film, or a layer). Therefore, another element may be providedbetween elements having a connection relation shown in drawings andtexts, without limitation on a predetermined connection relation, forexample, the connection relation shown in the drawings and the texts.

For example, in the case where X and Y are electrically connected, oneor more elements that enable electrical connection between X and Y(e.g., a switch, a transistor, a capacitor, an inductor, a resistor,and/or a diode) can be connected between X and Y. Note that theexpression “electrically connected” is sometimes used to mean“connected”. In this case, “electrically connected” has the meaning of“functionally connected” and “directly connected”.

For example, in the case where X and Y are functionally connected, oneor more circuits that enable functional connection between X and Y(e.g., a logic circuit such as an inverter, a NAND circuit, or a NORcircuit; a signal converter circuit such as a DA converter circuit, anAD converter circuit, or a gamma correction circuit; a potential levelconverter circuit such as a power supply circuit (e.g., a dc-dcconverter, a step-up dc-dc converter, or a step-down dc-dc converter) ora level shifter circuit for changing the potential level of a signal; avoltage source; a current source; a switching circuit; an amplifiercircuit such as a circuit that can increase signal amplitude, the amountof current, or the like, an operational amplifier, a differentialamplifier circuit, a source follower circuit, or a buffer circuit; asignal generation circuit; a memory circuit; and/or a control circuit)can be connected between X and Y. When a signal output from X istransmitted to Y, it can be said that X and Y are functionally connectedeven if another circuit is provided between X and Y.

For example, in this specification and the like, when it is explicitlydescribed that Y is formed on or over X, it does not necessarily meanthat Y is formed on and in direct contact with X. The descriptionincludes the case where X and Y are not in direct contact with eachother, that is, the case where another object is placed between X and Y.Here, each of X and Y corresponds to an object (e.g., a device, anelement, a circuit, a wiring, an electrode, a terminal, a conductivefilm, or a layer).

Accordingly, for example, when it is explicitly described that a layer Yis formed on (or over) a layer X, it includes both the case where thelayer Y is formed on and in direct contact with the layer X, and thecase where another layer (e.g., a layer Z) is formed on and in directcontact with the layer X and the layer Y is formed on and in directcontact with the layer Z. Note that another layer (e.g., the layer Z)may be a single layer or a plurality of layers (a stack).

Similarly, when it is explicitly described that Y is formed above X, itdoes not necessarily mean that Y is formed on and in direct contact withX, and another object may be placed between X and Y. Therefore, forexample, when it is described that a layer Y is formed above a layer X,it includes both the case where the layer Y is formed on and in directcontact with the layer X, and the case where another layer (e.g., alayer Z) is formed on and in direct contact with the layer X and thelayer Y is formed on and in direct contact with the layer Z. Note thatanother layer (e.g., the layer Z) may be a single layer or a pluralityof layers (a stack).

Note that when it is explicitly described that Y is formed over, on, orabove X, it includes the case where Y is formed obliquely over/above X.

Note that the same can be said when it is explicitly described that Y isformed below or under X.

For example, in this specification and the like, explicit singular formspreferably mean singular forms. However, the singular form can alsoinclude the plural without limitation to the above. Similarly, explicitplural forms preferably mean plural forms. However, the plural form caninclude the singular without limitation to the above.

For example, in this specification and the like, the terms “first”,“second,” “third,” and the like are used for distinguishing variouselements, members, regions, layers, and areas from each other.Therefore, the terms “first”, “second”, “third,” and the like do notlimit the number of the elements, members, regions, layers, areas, orthe like. Further, for example, “first” can be replaced with “second”,“third”, or the like.

For example, in this specification and the like, terms for describingspatial arrangement, such as “over”, “above”, “under”, “below”,“laterally”, “right”, “left”, “obliquely”, “behind”, “front”, “inside”,“outside”, and “in” are often used for briefly showing a relationbetween an element and another element or between a feature and anotherfeature with reference to a diagram. Note that embodiments of thepresent invention are not limited thereto, and such terms for describingspatial arrangement can indicate not only the direction illustrated in adiagram but also another direction. For example, when it is explicitlydescribed that Y is over X it does not necessarily mean that Y is placedover X. Since a device in a diagram can be inverted or rotated by 180°,the case where Y is placed under B can be included. Accordingly, “over”can refer to the direction described by “under” in addition to thedirection described by “over”. Note that embodiments of the presentinvention are not limited to this, and “over” can refer to any of theother directions described by “laterally”, “right”, “left”, “obliquely”,“behind”, “front”, “inside”, “outside”, “in”, and the like in additionto the directions described by “over” and “under” because the device inthe diagram can be rotated in a variety of directions. That is, theterms for describing spatial arrangement can be construed adequatelydepending on the situation.

Note that the size, the thickness of a layer, or a region in diagrams issometimes exaggerated for simplicity. Therefore, embodiments of thepresent invention are not limited to such scales.

Note that a diagram schematically illustrates an ideal example, andembodiments of the present invention are not limited to the shape,value, or the like illustrated in the diagram. For example, it ispossible to include variations in shape due to a manufacturing techniqueor an error, or variations in signal, voltage, or current due to noiseor difference in timing.

According to one embodiment of the present invention, higher performancecan be realized or the drive capability of a semiconductor device can beimproved.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIG. 1A illustrates an example of a circuit diagram of a semiconductordevice in Embodiment 1, and FIGS. 1B and 1C each illustrate an exampleof a schematic diagram for explaining operation of the semiconductordevice;

FIGS. 2A to 2C each illustrate an example of a schematic diagram forexplaining operation of a semiconductor device in Embodiment 1;

FIGS. 3A to 3D each illustrate an example of a circuit diagram of asemiconductor device in Embodiment 1;

FIGS. 4A to 4C each illustrate an example of a circuit diagram of asemiconductor device in Embodiment 1;

FIG. 5A illustrates an example of a circuit diagram of a semiconductordevice in Embodiment 2, and FIG. 5B illustrates an example of a timingchart for explaining operation of the semiconductor device;

FIGS. 6A to 6C each illustrate an example of a schematic diagram forexplaining operation of a semiconductor device in Embodiment 2;

FIGS. 7A to 7C each illustrate an example of a circuit diagram of asemiconductor device in Embodiment 2;

FIGS. 8A to 8C each illustrate an example of a circuit diagram of asemiconductor device in Embodiment 2;

FIGS. 9A and 9B each illustrate an example of a circuit diagram of asemiconductor device in Embodiment 2;

FIG. 10 illustrates an example of a circuit diagram of a semiconductordevice in Embodiment 2;

FIGS. 11A and 11B each illustrate an example of a circuit diagram of asemiconductor device in Embodiment 3;

FIGS. 12A and 12B each illustrate an example of a schematic diagram forexplaining operation of a semiconductor device in Embodiment 3;

FIGS. 13A and 13B each illustrate an example of a circuit diagram of asemiconductor device in Embodiment 3;

FIGS. 14A and 14B each illustrate an example of a schematic diagram forexplaining operation of a semiconductor device in Embodiment 3;

FIGS. 15A to 15D illustrate an example of a process for manufacturing asemiconductor device in Embodiment 5;

FIG. 16 illustrates an example of a circuit diagram of a semiconductordevice in Embodiment 4;

FIG. 17 illustrates an example of a circuit diagram of a semiconductordevice in Embodiment 4;

FIGS. 18A and 18B each illustrate an example of a block diagram of adisplay device in Embodiment 6;

FIGS. 19A to 19D each illustrate an example of a block diagram of adisplay device in Embodiment 6;

FIG. 20A illustrates an example of a circuit diagram of a pixel inEmbodiment 7, and FIG. 20B illustrates an example of a cross-sectionalview of the pixel;

FIGS. 21A to 21C each illustrate an example of a cross-sectional view ofa pixel in Embodiment 7;

FIGS. 22A to 22C each illustrate an example of a timing chart forexplaining operation of a pixel in Embodiment 7;

FIG. 23 illustrates an example of a semiconductor device in Embodiment5;

FIGS. 24A and 24B each illustrate an example of a semiconductor devicein Embodiment 5;

FIGS. 25A and 25B each illustrate an example of a semiconductor devicein Embodiment 5;

FIG. 26 illustrates an example of a semiconductor device in Embodiment5;

FIGS. 27A to 27H each illustrate an example of an electronic device inEmbodiment 8;

FIGS. 28A to 28H each illustrate an example of an electronic device inEmbodiment 8; and

FIGS. 29A to 29D illustrate an example of a process for manufacturing asemiconductor device in Embodiment 9.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments will be described below with reference to the accompanyingdrawings. Note that the embodiments can be carried out in many differentmodes, and it is easily understood by those skilled in the art thatmodes and details can be modified in various ways without departing fromthe purpose and the scope of the present invention. Therefore, thepresent invention is not interpreted as being limited to the descriptionof the embodiments. Note that in structures described below, the sameportions or portions having similar functions are denoted by the samereference numerals, and description thereof is not repeated.

Note that what is described in one embodiment (or part of the content)can be applied to, combined with, or replaced with content (or partthereof) in one or a plurality of embodiments (which may be this oneembodiment and/or other embodiments).

Embodiment 1

In this embodiment, an example of a semiconductor device and an exampleof a method for driving the semiconductor device will be described. Inparticular, an example of a circuit using bootstrap operation and amethod for driving the circuit will be described.

First, an example of a structure of a semiconductor device in thisembodiment will be described.

FIG. 1A illustrates an example of a semiconductor device in thisembodiment. The semiconductor device in FIG. 1A includes a transistor101 and a transistor 102. A first terminal of the transistor 101 isconnected to a wiring 111. A second terminal of the transistor 101 isconnected to a wiring 112. A first terminal of the transistor 102 isconnected to a wiring 113. A second terminal of the transistor 102 isconnected to a gate of the transistor 101. A gate of the transistor 102is connected to the wiring 113. Note that the semiconductor device inthis embodiment is not limited to having the structure illustrated inFIG. 1A and can have a variety of other structures.

Note that a portion where the gate of the transistor 101 and the secondterminal of the transistor 102 are connected is referred to as a node11.

Note that the case where the transistors 101 and 102 are n-channeltransistors is described below. An n-channel transistor is turned onwhen the potential difference between the gate and the source is higherthan the threshold voltage.

Note that an oxide semiconductor is preferably used for a semiconductorlayer of a transistor included in the semiconductor device in thisembodiment. The use of an oxide semiconductor for a semiconductor layercan realize improvement in subthreshold swing (S value) of thetransistor, reduction in off-state current of the transistor, and/orimprovement in withstand voltage of the transistor.

Note that in this specification and the like, it is sometimes possiblefor those skilled in the art to constitute one embodiment of theinvention even when portions to which all terminals of an active element(e.g., a transistor or a diode), a passive element (e.g., a capacitor ora resistor), or the like are connected are not specified. In particular,when there are likely to be a plurality of possible portions to beconnected to a terminal, it is not necessary to specify the portion tobe connected to the terminal. Therefore, it is sometimes possible toconstitute one embodiment of the invention by specifying only a portionto be connected to some of terminals of an active element (e.g., atransistor or a diode), a passive element (e.g., a capacitor or aresistor), or the like.

In addition, in this specification and the like, it is sometimespossible for those skilled in the art to specify the invention when atleast a portion to be connected to a terminal in a circuit is specified.Moreover, it is sometimes possible for those skilled in the art tospecify the invention when at least a function of a circuit isspecified. Therefore, when a portion to be connected to a terminal isspecified in a circuit, the circuit is disclosed as one embodiment ofthe invention even if a function is not specified, and one embodiment ofthe invention can be constituted. Furthermore, when a function of acircuit is specified, the circuit is disclosed as one embodiment of theinvention even if a portion to be connected to a terminal is notspecified, and one embodiment of the invention can be constituted.

Next, an example of potentials applied to each wiring will be described.

A potential V1 is applied to the wiring 111. That is, a constant voltage(e.g., a voltage V1) is supplied to the wiring 111. Alternatively, thepotential V1 and a potential V2 (the potential V2<the potential V1) areselectively or alternately applied to the wiring 111. That is, a signal(e.g., a clock signal) is input to the wiring 111. When the potential V1is applied to the wiring 111, the wiring 111 has a function of a powersupply line. On the other hand, when the potential V1 and the potentialV2 are selectively applied to the wiring 111, the wiring 111 has afunction of a signal line (e.g., a clock signal line). Note that thepotentials applied to the wiring 111 are not limited to the potential V1and the potential V2, and various other potentials can be applied to thewiring 111.

A signal is output from the wiring 112. Therefore, the wiring 112 has afunction of a signal line. Note that the potential of the wiring 112 isin the range from V2 to V1, for example.

The potential V1 and the potential V2 (the potential V2<the potentialV1) are selectively applied to the wiring 113. That is, a signal isinput to the wiring 113. Therefore, the wiring 113 has a function of asignal line. Note that the potentials applied to the wiring 113 are notlimited to the potential V1 and the potential V2. For example, aconstant voltage can be supplied to the wiring 113. As another example,an analog signal or a signal with three or more potentials can be inputto the wiring 113.

Description is given on the assumption that a node, a wiring, anelectrode, a terminal, or the like is supplied with a potential A andthus has a potential equal to the potential A.

Note that the terms “equal,” “same,” and the like in this specificationand the like sometimes mean that there is a difference within the marginof error. For example, in the case where potentials (or voltages) areequal to each other, the margin of error may be at least ±10%, ispreferably ±5%, and is more preferably ±3%. Alternatively, the margin oferror includes the range of change in potential due to leakage current,feedthrough, noise, or the like; the range of measurement error due to ameasurement device or the like; the range of variation in potential dueto process variation; or the like.

Next, an example of operation of the semiconductor device in thisembodiment will be described. The following two different operationswill be described below: the operation in the case where the potentialV1 is applied to the wiring 111, and the operation in the case where thepotential V1 and the potential V2 are selectively applied to the wiring111.

An example of the operation of the semiconductor device in thisembodiment in the case where the potential V1 is applied to the wiring111 is described.

It is assumed that an initial value of the potential of the node 11 andan initial value of the potential of the wiring 112 are equal to thepotential V2. When the potential V1 is applied to the wiring 113, thetransistor 102 is turned on, so that electrical continuity isestablished between the wiring 113 and the node 11. Then, the potentialof the wiring 113 is supplied to the node 11, so that the potential ofthe node 11 starts to rise. Then, the transistor 101 is turned on whenthe potential of the node 11 reaches V2+Vth101 (Vth101 represents thethreshold voltage of the transistor 101), so that electrical continuityis established between the wiring 111 and the wiring 112. Then, thepotential of the wiring 111 is supplied to the wiring 112, so that thepotential of the wiring 112 starts to rise (see FIG. 1B).

After that, the transistor 102 is turned off when the potential of thenode 11 reaches V1−Vth102 (Vth102 represents the threshold voltage ofthe transistor 102 and satisfies V2+Vth101<V1−Vth102); thus, electricalcontinuity between the wiring 113 and the node 11 is broken. Then, thenode 11 enters a floating state. After that, the potential of the wiring112 continues to rise, and then rises to a value equal to that of thepotential V1. With the rise in potential of the wiring 112, thepotential of the node 11 rises to V1+Vth101+Va (Va is a positive number)due to parasitic capacitance between the gate and the second terminal ofthe transistor 101 (see FIG. 1C). This is so-called bootstrap operation.

Note that when the potential V2 is applied to the wiring 113, thetransistor 102 is turned off, so that electrical continuity is notestablished between the wiring 113 and the node 11. That is, the node 11enters a floating state. In this case, the operation of thesemiconductor device in FIG. 1A depends on the potential of the node 11before the potential V2 is applied to the wiring 113. When the potentialV2 is applied to the wiring 113 on the assumption that the potential ofthe node 11 before the potential V2 is applied to the wiring 113 islower than V2+Vth101, for example, the transistor 101 is turned off, sothat electrical continuity is not established between the wiring 111 andthe wiring 112. Therefore, the potential of the wiring 112 remains thesame as that before the potential V2 is applied to the wiring 113. Onthe other hand, when the potential V2 is applied to the wiring 113 onthe assumption that the potential of the node 11 before the potential V2is applied to the wiring 113 exceeds V2+Vth101, for example, thetransistor 101 is turned on, so that electrical continuity isestablished between the wiring 111 and the wiring 112. Thus, thepotential of the wiring 112 becomes equal to the potential V1, and thepotential of the node 11 becomes V1+Vth101+Va due to bootstrapoperation.

An example of the operation of the semiconductor device in thisembodiment in the case where the potential V1 and the potential V2 areselectively applied to the wiring 111 is described.

It is assumed that an initial value of the potential of the node 11 andan initial value of the potential of the wiring 112 are equal to thepotential V2. When the potential V1 is applied to the wiring 113 and thepotential V2 is applied to the wiring 111, the transistor 102 is turnedon, so that electrical continuity is established between the wiring 113and the node 11. Then, the potential of the wiring 113 is supplied tothe node 11, so that the potential of the node 11 starts to rise. Then,the transistor 101 is turned on when the potential of the node 11reaches V2+Vth101, so that electrical continuity is established betweenthe wiring 111 and the wiring 112. Then, the potential of the wiring 111is supplied to the wiring 112, so that the potential of the wiring 112is equal to the potential V2 (see FIG. 2A).

After that, the transistor 102 is turned off when the potential of thenode 11 reaches V1−Vth102, so that electrical continuity between thewiring 113 and the node 11 is broken. Then, the node 11 enters afloating state (see FIG. 2B).

After that, the potential V1 is applied to the wiring 111. At this time,the node 11 remains in a floating state, so that the potential of thenode 11 remains V1−Vth102. Therefore, the transistor 101 remains on andelectrical continuity between the wiring 111 and the wiring 112 remainsestablished. That is, the potential of the wiring 111 continues to besupplied to the wiring 112. Accordingly, the potential of the wiring 112starts to rise at the same time as the application of the potential V1to the wiring 111, and rises to a value equal to that of the potentialV1. With the rise in potential of the wiring 112, the potential of thenode 11 rises to V1+Vth101+Va (Va is a positive number) due to parasiticcapacitance between the gate and the second terminal of the transistor101 (see FIG. 2C). This is so-called bootstrap operation.

Note that when the potential V2 is applied to the wiring 113, thetransistor 102 is turned off, so that electrical continuity is notestablished between the wiring 113 and the node 11. That is, the node 11enters a floating state. In this case, the operation of thesemiconductor device in FIG. 2A depends on the potential of the node 11before the potential V2 is applied to the wiring 113. When the potentialV2 is applied to the wiring 113 on the assumption that the potential ofthe node 11 before the potential V2 is applied to the wiring 113 islower than V2+Vth101, for example, the transistor 101 is turned off, sothat electrical continuity is not established between the wiring 111 andthe wiring 112. Therefore, the potential of the wiring 112 remains thesame as that before the potential V2 is applied to the wiring 113. Onthe other hand, when the potential V2 is applied to the wiring 113 onthe assumption that the potential of the node 11 before the potential V2is applied to the wiring 113 exceeds V2+Vth101, for example, thetransistor 101 is turned on, so that electrical continuity isestablished between the wiring 111 and the wiring 112. Thus, thepotential of the wiring 112 becomes equal to the potential of the wiring111. That is, the potential of the wiring 112 becomes equal to thepotential V1 when the potential V1 is applied to the wiring 111, whereasthe potential of the wiring 112 becomes equal to the potential V2 whenthe potential V2 is applied to the wiring 111.

As described above, in the semiconductor device in this embodiment, thepotential of the wiring 112 can be made equal to the potential of thewiring 111 by using the bootstrap operation.

In a conventional semiconductor device, a transistor has a large Svalue. For that reason, it takes a long time after the potential V1 isapplied to the wiring 113 until the transistor 102 is turned off; thetiming at which the potential of the node 11 starts to rise due tobootstrap operation is delayed; the potential of the node 11 is lowered;the potential difference between the gate and the second terminal of thetransistor 101 is small; the rise time of the potential of the wiring112 is long; a load capable of being connected to the wiring 112 issmall; the channel width of the transistor 101 is large; or the layoutarea is large.

In contrast, the S value can be reduced in the semiconductor device ofthis embodiment because an oxide semiconductor is used for asemiconductor layer of a transistor included in the semiconductordevice. For that reason, the drive capability of the semiconductordevice can be improved. For example, when the transistor 102 has a smallS value, it is possible to shorten the time after the potential V1 isapplied to the wiring 113 until the transistor 102 is turned off; thus,the timing at which the potential of the node 11 starts to rise due tobootstrap operation can be advanced. When the timing at which thepotential of the node 11 starts to rise is advanced, the potential ofthe node 11 can be made higher, so that the potential difference betweenthe gate and the second terminal of the transistor 101 can be increased.As a result, the rise time of the potential of the wiring 112 can beshortened. Alternatively, even when a large load is connected to thewiring 112, the load can be driven. Alternatively, the channel width ofthe transistor 101 can be reduced, so that the layout area can bedecreased. As another example, a small S value of the transistor 101 canshorten the rise time of the potential of the wiring 112.

Further, in a conventional semiconductor device, the off-state currentof a transistor is large. For that reason, the amount of electric chargethat is lost from the node 11 over time is large; the potential of thenode 11 is lowered; the time during which the potential of the node 11can be kept higher than V1+Vth101 is short; it is difficult to lower thedrive frequency; or the range of drive frequencies with which thesemiconductor device can operate is narrow.

In contrast, the off-state current of the transistor included in thesemiconductor device of this embodiment is small. Thus, the drivecapability of the semiconductor device can be improved. For example,when the transistor 102 has a small off-state current, it is possible todecrease the amount of electric charge lost from the node 11, so that areduction in potential of the node 11 can be suppressed. In other words,it is possible to extend the time during which the potential of the node11 can be kept higher than V1+Vth101. As a result, the drive frequencycan be lowered, so that the range of drive frequencies with which thesemiconductor device in this embodiment can operate can be enlarged.

In the case where the potential V1 and the potential V2 are selectivelyapplied to the wiring 111, the potential of the node 11 rises toV1−Vth102 and then further rises due to bootstrap operation. In otherwords, the potential difference between the gate and the second terminalof the transistor 101 can be increased. Thus, the rise time of thepotential of the wiring 112 can be shortened. Alternatively, even when alarge load is connected to the wiring 112, the load can be driven.Alternatively, the channel width of the transistor 101 can be reduced,so that the layout area can be decreased.

Note that in the case where the potential V1 and the potential V2 areselectively applied to the wiring 111, after the potential V1 is appliedto the wiring 113, the potential of the wiring 112 rises at the sametime as the application of the potential V1 to the wiring 111.Therefore, the semiconductor device in this embodiment can be used aspart of a shift register circuit.

Next, an example of a function of each transistor will be described.

The transistor 101 has a function of controlling electrical continuitybetween the wiring 111 and the wiring 112. That is, the transistor 101has a function of a switch. Alternatively, the transistor 101 has afunction of controlling the timing of supplying the potential of thewiring 111 to the wiring 112, a function of controlling the timing ofraising the potential of the wiring 112, or a function of controllingthe timing of raising the potential of the node 11 by parasiticcapacitance between the gate and the second terminal of the transistor101. Note that the transistor 101 has at least one of the abovefunctions.

The transistor 102 has a function of controlling electrical continuitybetween the wiring 113 and the node 11. That is, the transistor 102 hasa function of a switch. Alternatively, the transistor 102 has a functionof establishing electrical continuity between the wiring 113 and thenode 11 when the potential of the wiring 113 is higher than thepotential of the node 11, and breaking electrical continuity between thewiring 113 and the node 11 when the potential of the wiring 113 is lowerthan the potential of the node 11. That is, the transistor 102 has afunction of a diode. Alternatively, the transistor 102 has a function ofcontrolling the timing of supplying the potential of the wiring 113 tothe node 11, a function of controlling the timing of raising thepotential of the node 11, or a function of controlling the timing ofbringing the node 11 into a floating state. Note that the transistor 102has at least one of the above functions.

Next, an example of potentials applied to each wiring will be described.By controlling a potential applied to each wiring as appropriate, thesemiconductor device in this embodiment can have a variety of functionsor the drive capability of the semiconductor device can be improved.

For example, in the case where the potential V1 and the potential V2 areselectively applied to the wiring 113, the wiring 111 can be suppliedwith a potential higher than the potential V1 or a potential lower thanthe potential V1. When the wiring 111 is supplied with a potential thatis higher or lower than the potential V1, the semiconductor device inthis embodiment can have a function of a level shift circuit.

Note that when the wiring 111 is supplied with a potential higher thanthe potential V1, it is preferable that the potential applied to thewiring 111 be higher than the potential V1 and be 4 times or less ashigh as the potential V1. More preferably, the potential is 1.2 to 3times as high as the potential V1. Further preferably, the potential is1.5 to 2.3 times as high as the potential V1.

Note that when the wiring 111 is supplied with a potential lower thanthe potential V1, it is preferable that the potential applied to thewiring 111 be 0.2 times or more as high as the potential V1 and be lessthan the potential V1. More preferably, the potential is 0.3 to 0.9times as high as the potential V1. Further preferably, the potential is0.5 to 0.7 times as high as the potential V1.

For example, in the case where the potential V1 is applied to the wiring111, the wiring 113 can be supplied with a potential higher than thepotential V1. Specifically, it is preferable that the potential appliedto the wiring 113 be higher than the potential applied to the wiring 111and be 3 times or less as high as the potential applied to the wiring111. The potential applied to the wiring 113 is more preferably 1.3 to2.5 times, further preferably 1.5 to 2 times as high as the potentialapplied to the wiring 111. When the potential applied to the wiring 113is high, it is possible to shorten the time after the potential V1 isapplied to the wiring 113 until the transistor 102 is turned off; thus,the timing at which the potential of the node 11 starts to rise due tobootstrap operation can be advanced. When the timing at which thepotential of the node 11 starts to rise is advanced, the potential ofthe node 11 can be made higher, so that the potential difference betweenthe gate and the second terminal of the transistor 101 can be increased.As a result, the rise time of the potential of the wiring 112 can beshortened. Alternatively, even when a large load is connected to thewiring 112, the load can be driven. Alternatively, the channel width ofthe transistor 101 can be reduced, so that the layout area can bedecreased.

Next, an example of the threshold voltage of each transistor will bedescribed. When each transistor has an appropriate threshold voltage,the drive capability of the semiconductor device can be improved.

For example, it is preferable that the threshold voltage of thetransistor 102 be as low as possible. Specifically, the thresholdvoltage of the transistor 102 is preferably lower than that of thetransistor 101. The threshold voltage of the transistor 102 ispreferably 0.1 times or more as high as that of the transistor 101 andlower than that of the transistor 101. The threshold voltage of thetransistor 102 is more preferably 0.3 to 0.9 times, further preferably0.5 to 0.7 times as high as that of the transistor 101. When thetransistor 102 has a low threshold voltage, it is possible to shortenthe time after the potential V1 is applied to the wiring 113 until thetransistor 102 is turned off; thus, the timing at which the potential ofthe node 11 starts to rise due to bootstrap operation can be advanced.When the timing at which the potential of the node 11 starts to rise isadvanced, the potential of the node 11 can be made higher, so that thepotential difference between the gate and the second terminal of thetransistor 101 can be increased. As a result, the rise time of thepotential of the wiring 112 can be shortened. Alternatively, even when alarge load is connected to the wiring 112, the load can be driven.Alternatively, the channel width of the transistor 101 can be reduced,so that the layout area can be decreased.

Moreover, for example, the threshold voltage of the transistor 101 ispreferably lower than the drive voltage of the semiconductor device(e.g., the potential V1−the potential V2). Specifically, the thresholdvoltage of the transistor 101 is preferably 1/50 to ½ times, morepreferably 1/40 to 1/7 times, further preferably 1/30 to 1/10 times ashigh as the drive voltage of the semiconductor device. The thresholdvoltage of the transistor 101 is made lower than the drive voltage ofthe semiconductor device, so that a malfunction of the semiconductordevice can be prevented and the semiconductor device can operatecorrectly.

Next, an example of the size of each transistor will be described. Wheneach transistor has an appropriate size, the drive capability of thesemiconductor device in this embodiment can be improved.

For example, the channel width of the transistor 101 is preferablylarger than that of the transistor 102. The channel width of thetransistor 101 is preferably 2 to 100 times, more preferably 5 to 50times, further preferably 10 to 30 times as large as that of thetransistor 102.

Note that the current supply capability of a transistor can becontrolled by the channel width (W) of the transistor. Specifically, thelarger the channel width of the transistor is, the more the currentsupply capability of the transistor is improved. Note that a factor incontrolling the current supply capability of a transistor is not limitedto the channel width of the transistor. For example, the current supplycapability of a transistor can be controlled by the channel length (L)of the transistor, the W/L ratio of the transistor, a potentialdifference (Vgs) between a gate and a source of the transistor, or thelike. Specifically, the current supply capability of a transistor isimproved as the channel length of the transistor is smaller, the W/Lratio of the transistor is larger, or Vgs of the transistor is larger.Therefore, in this specification and the like, the expression “thechannel width of the transistor is large” has the same meaning as theexpressions “the channel length of the transistor is small,” “the W/Lratio of the transistor is large,” and “Vgs of the transistor is large”.

Next, a semiconductor device with a structure different from that of thesemiconductor device in FIG. 1A will be described.

For example, in the semiconductor device illustrated in FIG. 1A, awiring to which the gate and/or the first terminal of the transistor 102is/are connected is not limited to the wiring 113 and can be variousother wirings.

FIG. 3A illustrates an example of the case where the gate of thetransistor 102 is connected to the wiring 111 in the semiconductordevice illustrated in FIG. 1A. In the semiconductor device in FIG. 3A,in the case where the potential V2 is applied to the wiring 113, thepotential of the node 11 can be made equal to the potential V2.

FIG. 3B illustrates an example of the case where the first terminal ofthe transistor 102 is connected to the wiring 111 in the semiconductordevice illustrated in FIG. 1A. In the semiconductor device in FIG. 3B,the potential V1 can be supplied to the node 11 from the wiring 111whose resistance is lower than that of the wiring 113, so that thepotential of the node 11 can be raised quickly.

FIG. 3C illustrates an example of the case where the gate of thetransistor 102 is connected to the wiring 111 and the first terminal ofthe transistor 102 is connected to the wiring 111 in the semiconductordevice illustrated in FIG. 1A. In the semiconductor device in FIG. 3C,the wiring 113 can be omitted, so that the number of wirings and thenumber of signals can be reduced.

In addition, for example, when the capacitance between the node 11 andthe wiring 112 is increased in the semiconductor devices illustrated inFIG. 1A and FIGS. 3A to 3C, the potential of the node 11 at the time ofbootstrap operation can be made higher.

Specifically, for example, in the semiconductor devices illustrated inFIG. 1A and FIGS. 3A to 3C, a capacitor can be connected between thenode 11 and the wiring 112. It is preferable that one electrode of acapacitor 121 be formed using the material used for the gate of thetransistor 101 and be connected to the node 11. Moreover, it ispreferable that the other electrode of the capacitor 121 be formed usingthe material used for the second terminal of the transistor 101 and beconnected to the wiring 112. A contact hole or the like can be omittedin such a manner, so that the layout area can be decreased.

Alternatively, for example, in the semiconductor devices illustrated inFIG. 1A and FIGS. 3A to 3C, the area where the material used for formingthe gate of the transistor 101 overlaps with the material used forforming the second terminal of the transistor 101 can be made largerthan the area where the material used for forming the gate of thetransistor 101 overlaps with the material used for forming the firstterminal of the transistor 101. Specifically, it is preferable that thearea where the material of the gate of the transistor 101 overlaps withthe material of the second terminal of the transistor 101 be larger thanthe area where the material of the gate of the transistor 101 overlapswith the material of the first terminal of the transistor 101, and be 5times or less as large as the area where the material of the gate of thetransistor 101 overlaps with the material of the first terminal of thetransistor 101. The area where the material of the gate of thetransistor 101 overlaps with the material of the second terminal of thetransistor 101 is more preferably 1.5 to 4 times, further preferably 2to 3 times as large as the area where the material of the gate of thetransistor 101 overlaps with the material of the first terminal of thetransistor 101.

FIG. 3D illustrates an example of the case where the capacitor 121 isconnected between the gate and the second terminal of the transistor 101in the semiconductor device illustrated in FIG. 1A.

For example, in the semiconductor devices in FIGS. 3A to 3D, the firstterminal of the transistor 101 and the gate or the first terminal of thetransistor 102 can be connected to different wirings.

FIG. 4A illustrates an example of the case where the first terminal ofthe transistor 101 is connected to a wiring 111A and the gate of thetransistor 102 is connected to a wiring 111B in the semiconductor deviceillustrated in FIG. 3A.

FIG. 4B illustrates an example of the case where the first terminal ofthe transistor 101 is connected to the wiring 111A and the firstterminal of the transistor 102 is connected to the wiring 111B in thesemiconductor device illustrated in FIG. 3B.

FIG. 4C illustrates an example of the case where the first terminal ofthe transistor 101 is connected to the wiring 111A and the gate and thefirst terminal of the transistor 102 are connected to the wiring 111B inthe semiconductor device illustrated in FIG. 3C.

Note that the wirings 111A and 111B have a function similar to that ofthe wiring 111. The potential V1 is supplied to the wirings 111A and111B. Note that potentials applied to the wirings 111A and 111B can bedifferent from each other. For example, when the wiring 111A is suppliedwith a potential higher than the potential V1 or a potential lower thanthe potential V1, the semiconductor devices illustrated in FIGS. 4A to4C can have a function of a level shift circuit. As another example,when the wiring 111B is supplied with a potential higher than thepotential V1, the rise time of the potential of the node 11 can beshortened. On the other hand, when the wiring 111B is supplied with apotential lower than the potential V1, the timing at which thetransistor 102 is turned off can be advanced.

Note that in this specification and the like, a transistor with amulti-gate structure having two or more gate electrodes can be used as atransistor, for example. In the multi-gate structure, a plurality ofchannel regions corresponding to a plurality of gate electrodes areconnected in series, so that the structure is such that a plurality oftransistors are connected in series. For that reason, with themulti-gate structure, the off-state current can be further reduced andthe withstand voltage of the transistor can be increased (thereliability can be improved). Alternatively, with the multi-gatestructure, drain-source current does not change much even ifdrain-source voltage changes when the transistor operates in asaturation region, so that a flat slope of voltage-currentcharacteristics can be obtained. By utilizing the flat slope of thevoltage-current characteristics, an ideal current source circuit or anactive load having an extremely large resistance can be realized. As aresult, a differential circuit, a current mirror circuit, or the likewith excellent properties can be realized.

Note that a transistor with a structure where gate electrodes are formedabove and below a channel can be used as a transistor, for example. Withthe structure where the gate electrodes are formed above and below thechannel, a circuit structure where a plurality of transistors areconnected in parallel is provided. Thus, a channel region is increased,so that the amount of current can be increased. Alternatively, with thestructure where gate electrodes are formed above and below the channel,a depletion layer is easily formed; thus, the S value can be improved.

Note that as a transistor, for example, it is possible to use atransistor with a structure where a gate electrode is formed above achannel region, a structure where a gate electrode is formed below achannel region, a staggered structure, an inverted staggered structure,a structure where a channel region is divided into a plurality ofregions, a structure where channel regions are connected in parallel orin series, or the like.

Note that for example, a transistor with a structure where a sourceelectrode or a drain electrode overlaps with a channel region (or partthereof) can be used as a transistor. By using the structure where thesource electrode or the drain electrode overlaps with the channel region(or part thereof), unstable operation due to accumulation of electriccharge in part of the channel region can be prevented.

Note that in this specification and the like, in a diagram or a textdescribed in one embodiment, it is possible to take out part of thediagram or the text and constitute an embodiment of the invention.Therefore, in the case where a diagram or a text related to a certainportion is described, the context taken out from part of the diagram orthe text is also disclosed as one embodiment of the invention and canconstitute one embodiment of the invention. Thus, for example, in adiagram or a text including one or more of active elements (e.g.,transistors or diodes), wirings, passive elements (e.g., capacitors orresistors), conductive layers, insulating layers, semiconductor layers,organic materials, inorganic materials, components, devices, operatingmethods, manufacturing methods, or the like, it is possible to take outpart of the diagram or the text and constitute one embodiment of theinvention. For example, from a circuit diagram in which N circuitelements (e.g., transistors or capacitors; N is an integer) areprovided, it is possible to constitute one embodiment of the inventionby taking out M circuit elements (e.g., transistors or capacitors; M isan integer, where M<N). As another example, it is possible to constituteone embodiment of the invention by taking out M layers (M is an integer,where M<N) from a cross-sectional view in which N layers (N is aninteger) are provided. As another example, it is possible to constituteone embodiment of the invention by taking out M elements (M is aninteger, where M<N) from a flow chart in which N elements (N is aninteger) are provided.

Note that in the case where at least one specific example is describedin a diagram or a text described in one embodiment in this specificationand the like, it will be readily appreciated by those skilled in the artthat a broader concept of the specific example can be derived.Therefore, in the case where at least one specific example is describedin the diagram or the text described in one embodiment, a broaderconcept of the specific example is disclosed as one embodiment of theinvention, and one embodiment of the invention can be constituted.

Note that in this specification and the like, a content described in atleast a diagram (which may be part of the diagram) is disclosed as oneembodiment of the invention, and one embodiment of the invention can beconstituted. Therefore, when a certain content is described in adiagram, the content is disclosed as one embodiment of the inventioneven when the content is not described with a text, and one embodimentof the invention can be constituted. In a similar manner, part of adiagram, which is taken out from the diagram, is disclosed as oneembodiment of the invention, and one embodiment of the invention can beconstituted.

Embodiment 2

In this embodiment, an example of a structure of a semiconductor deviceand an example of a method for driving the semiconductor device will bedescribed. In particular, an example of an inverter circuit and a buffercircuit that include the semiconductor device shown in Embodiment 1 andan example of a method for driving the inverter circuit and the buffercircuit will be described.

First, an example of a structure of a semiconductor device in thisembodiment will be described.

FIG. 5A illustrates an example of a structure of a semiconductor device.The semiconductor device illustrated in FIG. 5A includes the transistor101, the transistor 102, a transistor 103, a transistor 104, the wiring112, the wiring 113, a wiring 114, and a wiring 115. The transistors 101to 104 are formed using an oxide semiconductor material. The transistors103 and 104 are n-channel transistors.

FIG. 5A illustrates the semiconductor device obtained by additionallyproviding the transistors 103 and 104 in the semiconductor device inFIG. 1A. A gate of the transistor 103 is connected to the wiring 114. Afirst terminal of the transistor 103 is connected to the wiring 115. Asecond terminal of the transistor 103 is connected to the wiring 112. Agate of the transistor 104 is connected to the wiring 114. A firstterminal of the transistor 104 is connected to the wiring 115. A secondterminal of the transistor 104 is connected to the gate of thetransistor 101. Note that the semiconductor device in this embodiment isnot limited to having the structure illustrated in FIG. 5A and can havea variety of other structures.

Next, an example of potentials applied to each wiring will be described.

The potential V1 and the potential V2 are selectively applied to thewiring 114. That is, a signal is input to the wiring 114. Therefore, thewiring 114 has a function of a signal line. Assuming that an invertedsignal of a signal input to the wiring 113 is input to the wiring 114,the potential V1 is applied to the wiring 114 when the potential V2 isapplied to the wiring 113, whereas the potential V2 is applied to thewiring 114 when the potential V1 is applied to the wiring 113. Note thatthe wirings 113 and 114 can be supplied with the same potential withoutlimitation to the above.

The potential V2 is applied to the wiring 115. That is, a constantvoltage (e.g., a voltage V2) is supplied to the wiring 115. Therefore,the wiring 115 has a function of a power supply line. Note that thepotential applied to the wiring 115 is not limited to the potential V2,and various other potentials can be applied to the wiring 115. Forexample, the potential V1 and the potential V2 can be selectivelyapplied to the wiring 115. When the potential V1 is applied to thewiring 115, a reverse bias can be applied to the transistors 103 and104; thus, shift of the threshold voltage of the transistors 103 and 104can be suppressed.

Next, an example of operation of the semiconductor device illustrated inFIG. 5A will be described. The following two different operations willbe described below: the operation in the case where the potential V2 isapplied to the wiring 113 and the potential V1 is applied to the wiring114, and the operation in the case where the potential V1 is applied tothe wiring 113 and the potential V2 is applied to the wiring 114.

FIG. 5B is an example of a timing chart for explaining operation of thesemiconductor device in FIG. 5A. FIG. 5B illustrates the potential ofthe wiring 113 (a potential V113), the potential of the wiring 114 (apotential V114), the potential of the node 11 (a potential V11), and thepotential of the wiring 112 (a potential V112).

First, the operation in the case where the potential V2 is applied tothe wiring 113 and the potential V1 is applied to the wiring 114 will bedescribed.

When the potential V2 is applied to the wiring 113 and the potential V1is applied to the wiring 114, the transistor 104 is turned on, so thatelectrical continuity is established between the wiring 115 and the node11. At this time, the transistor 102 is turned off, so that electricalcontinuity is not established between the wiring 113 and the node 11.The potential of the wiring 115 is supplied to the node 11 in such amanner, so that the potential of the node 11 becomes equal to thepotential V2. Thus, the transistor 101 is turned off, so that electricalcontinuity is not established between the wiring 111 and the wiring 112.At this time, the transistor 103 is turned on, so that electricalcontinuity is established between the wiring 115 and the wiring 112. Thepotential of the wiring 115 is supplied to the wiring 112 in such amanner, whereby the potential of the wiring 112 becomes equal to thepotential V2 (see FIG. 6A).

Then, the operation in the case where the potential V1 is applied to thewiring 113 and the potential V2 is applied to the wiring 114 will bedescribed.

When the potential V1 is applied to the wiring 113 and the potential V2is applied to the wiring 114, the transistor 104 is turned off, so thatelectrical continuity is not established between the wiring 115 and thenode 11. At this time, the transistor 102 is turned on, so thatelectrical continuity is established between the wiring 113 and the node11. The potential of the wiring 113 is supplied to the node 11 in such amanner, so that the potential of the node 11 starts to rise. Then, thepotential of the node 11 rises to V2+Vth101. Thus, the transistor 101 isturned on, so that electrical continuity is established between thewiring 111 and the wiring 112. At this time, the transistor 103 isturned off, so that electrical continuity is not established between thewiring 115 and the wiring 112. The potential of the wiring 111 issupplied to the wiring 112 in such a manner, whereby the potential ofthe wiring 112 starts to rise (see FIG. 6B).

After that, the potential of the node 11 rises to V1−Vth102. Thus, thetransistor 102 is turned off, so that electrical continuity between thewiring 113 and the node 11 is broken. Then, the node 11 enters afloating state. At this time, the potential of the wiring 112 continuesto rise. For that reason, the potential of the node 11 rises toV1+Vth101+Va because of parasitic capacitance between the gate and thesecond terminal of the transistor 101. This is so-called bootstrapoperation. Accordingly, the potential of the wiring 112 rises to a valueequal to that of the potential V1 (see FIG. 6C).

As described above, in the semiconductor device in this embodiment, thepotential of the wiring 112 can be made equal to the potential of thewiring 111 or the potential of the wiring 115 by using the bootstrapoperation.

In a conventional semiconductor device, a transistor has a large Svalue. For that reason, it takes a long time after the potential V1 isapplied to the wiring 113 until the transistor 102 is turned off; thetiming at which the potential of the node 11 starts to rise due tobootstrap operation is delayed; the potential of the node 11 is lowered;the potential difference between the gate and the second terminal of thetransistor 101 is small; the rise time of the potential of the wiring112 is long; a load capable of being connected to the wiring 112 issmall; the channel width of the transistor 101 is large; the layout areais large; the fall time of the potential of the wiring 112 is long; ittakes a long time after the potential V1 is applied to the wiring 114until the transistor 101 is turned off; it takes a long time for currentto flow from the wiring 111 to the wiring 115 through the transistor 101and the transistor 103; or power consumption is increased.

In contrast, the S value can be reduced in the semiconductor device ofthis embodiment because an oxide semiconductor is used for asemiconductor layer of a transistor included in the semiconductordevice. For that reason, the drive capability of the semiconductordevice can be improved. For example, when the transistor 102 has a smallS value, it is possible to shorten the time after the potential V1 isapplied to the wiring 113 until the transistor 102 is turned off; thus,the timing at which the potential of the node 11 starts to rise becauseof bootstrap operation can be advanced. When the timing at which thepotential of the node 11 starts to rise is advanced, the potential ofthe node 11 can be made higher, so that the potential difference betweenthe gate and the second terminal of the transistor 101 can be increased.As a result, the rise time of the potential of the wiring 112 can beshortened. Alternatively, even when a large load is connected to thewiring 112, the load can be driven. Alternatively, the channel width ofthe transistor 101 can be reduced, so that the layout area can bedecreased. As another example, a small S value of the transistor 101 canshorten the rise time of the potential of the wiring 112. As anotherexample, a small S value of the transistor 103 can shorten the fall timeof the potential of the wiring 112. As another example, a small S valueof the transistor 104 can shorten the time after the potential V1 isapplied to the wiring 114 until the transistor 101 is turned off.Accordingly, it is possible to suppress current flowing from the wiring111 to the wiring 115 through the transistor 101 and the transistor 103.Thus, power consumption can be reduced.

Further, in a conventional semiconductor device, the off-state currentof a transistor is large. For that reason, the amount of electric chargeleaking from the node 11 is large; the potential of the node 11 islowered; the time during which the potential of the node 11 can be kepthigher than V1+Vth101 is short; it is difficult to lower the drivefrequency; or the range of drive frequencies with which thesemiconductor device can operate is narrow.

In contrast, the off-state current of a transistor included in thesemiconductor device of this embodiment is small. For that reason, thedrive capability of the semiconductor device can be improved. Forexample, when the transistors 102 and 104 have a small off-statecurrent, it is possible to decrease the amount of electric chargeleaking from the node 11. Thus, a reduction in potential of the node 11can be suppressed. In other words, it is possible to extend the timeduring which the potential of the node 11 can be kept higher thanV1+Vth101. As a result, the drive frequency can be lowered, so that therange of drive frequencies with which the semiconductor device in thisembodiment can operate can be enlarged.

Note that a signal output from the wiring 112 is an inverted signal of asignal input to the wiring 114. That is, the semiconductor device inthis embodiment can have a function of an inverter circuit.Alternatively, a signal output from the wiring 112 is a non-invertedsignal of a signal input to the wiring 113. That is, the semiconductordevice in this embodiment can have a function of a buffer circuit.

Next, an example of a function of each transistor will be described.

The transistor 103 has a function of controlling electrical continuitybetween the wiring 115 and the wiring 112. That is, the transistor 103has a function of a switch. Alternatively, the transistor 103 has afunction of controlling the timing of supplying the potential of thewiring 115 to the wiring 112 or a function of controlling the timing ofdecreasing the potential of the wiring 112. Note that the transistor 103has at least one of the above functions.

The transistor 104 has a function of controlling electrical continuitybetween the wiring 115 and the node 11. That is, the transistor 104 hasa function of a switch. Alternatively, the transistor 104 has a functionof controlling the timing of supplying the potential of the wiring 115to the node 11 or a function of controlling the timing of decreasing thepotential of the node 11. Note that the transistor 104 has at least oneof the above functions.

Next, various potentials applied to each wiring will be described. Bycontrolling a potential applied to each wiring as appropriate, thesemiconductor device in this embodiment can have a variety of functionsor the drive capability of the semiconductor device can be improved.

For example, in the case where the potential V1 and the potential V2 areselectively applied to the wiring 113 or the wiring 114, the wiring 111can be supplied with a potential higher than the potential V1 or apotential lower than the potential V1. Thus, the semiconductor device inthis embodiment can have a function of a level shift circuit.

Note that when the wiring 111 is supplied with a potential higher thanthe potential V1, it is preferable that the potential be higher than thepotential V1 and be 4 times or less as high as the potential V1. Morepreferably, the potential is 1.2 to 3 times as high as the potential V1.Further preferably, the potential is 1.5 to 2.3 times as high as thepotential V1.

Note that when the wiring 111 is supplied with a potential lower thanthe potential V1, it is preferable that the potential be 0.2 times ormore as high as the potential V1 and be less than the potential V1. Morepreferably, the potential is 0.3 to 0.9 times as high as the potentialV1. Further preferably, the potential is 0.5 to 0.7 times as high as thepotential V1.

In addition, for example, in the case where the potential V1 and thepotential V2 are selectively applied to the wiring 114, a potentiallower than the potential V1 and a potential higher than the potential V2can be selectively applied to the wiring 113. In that case, the risetime of the potential of the wiring 114 is often shorter than that ofthe potential of the wiring 113. Alternatively, the fall time of thepotential of the wiring 114 is often shorter than that of the potentialof the wiring 113. The wiring 114 is connected to the wiring 113 throughan inverter circuit in many cases.

For example, in the case where the potential V1 and the potential V2 areselectively applied to the wiring 113, a potential lower than thepotential V1 and a potential higher than the potential V2 can beselectively applied to the wiring 114. In that case, the rise time ofthe potential of the wiring 113 is often shorter than that of thepotential of the wiring 114. Alternatively, the fall time of thepotential of the wiring 113 is often shorter than that of the potentialof the wiring 114. The wiring 113 is connected to the wiring 114 throughan inverter circuit in many cases.

Next, an example of the threshold voltage of each transistor will bedescribed. When each transistor has an appropriate threshold voltage,the drive capability of the semiconductor device can be improved.

For example, the threshold voltage of the transistor 103 is preferablyhigher than that of the transistor 101 and/or that of the transistor102. In particular, the threshold voltage of the transistor 103 ispreferably higher than that of the transistor 101 and is preferably 3times or less as high as that of the transistor 101. The thresholdvoltage of the transistor 103 is more preferably 1.2 to 2.5 times,further preferably 1.5 to 2 times as high as that of the transistor 101.

Furthermore, for example, the threshold voltage of the transistor 104 ispreferably higher than that of the transistor 101 and/or that of thetransistor 102. In particular, the threshold voltage of the transistor104 preferably exceeds that of the transistor 101 and is 3 times or lessas high as that of the transistor 101. The threshold voltage of thetransistor 104 is more preferably 1.2 to 2.5 times, further preferably1.5 to 2 times as high as that of the transistor 101.

For example, the sum of the threshold voltage of the transistor 101 andthe threshold voltage of the transistor 103 is preferably lower than thedrive voltage of the semiconductor device (e.g., the potential V1−thepotential V2). Specifically, the sum of the threshold voltages of thetransistors 101 and 103 is preferably 1/100 to ½ times, more preferably1/50 to ⅕ times, further preferably 1/30 to 1/10 times as high as thedrive voltage of the semiconductor device. The sum of the thresholdvoltages of the transistors 101 and 103 is made lower than the drivevoltage of the semiconductor device, so that a malfunction of thesemiconductor device can be prevented and the semiconductor device canoperate correctly.

Next, an example of the size of each transistor will be described. Wheneach transistor has an appropriate size, the drive capability of thesemiconductor device in this embodiment can be improved.

For example, the potential difference between the gate and the source ofthe transistor 101 when the transistor 101 is turned on is often smallerthan that between the gate and the source of the transistor 103 when thetransistor 103 is turned on. Therefore, the channel width of thetransistor 101 is preferably larger than that of the transistor 103.Specifically, the channel width of the transistor 101 is preferablylarger than that of the transistor 103 and is preferably 10 times orless as large as that of the transistor 103. The channel width of thetransistor 101 is more preferably 1.3 to 5 times, further preferably 1.5to 3 times as large as that of the transistor 103.

For example, the load of the wiring 112 is often larger than the load ofthe node 11. Therefore, the channel width of the transistor 103 ispreferably larger than that of the transistor 104. Specifically, thechannel width of the transistor 103 is preferably larger than that ofthe transistor 104 and is preferably 10 times or less as large as thatof the transistor 104. The channel width of the transistor 103 is morepreferably 1.5 to 7 times, further preferably 2 to 5 times as large asthat of the transistor 104.

For example, the channel length of the transistor 103 and/or the channellength of the transistor 104 is/are preferably large. Specifically, thechannel length of the transistor 103 is preferably larger than that ofthe transistor 101 and/or that of the transistor 102. Alternatively, thechannel length of the transistor 104 is preferably larger than that ofthe transistor 101 and/or that of the transistor 102. When the channellength of the transistor 103 and/or the transistor 104 is increased, theamount of shift in threshold voltage of the transistor 103 and/or thetransistor 104 can be reduced. Thus, the reliability of thesemiconductor device can be improved.

Next, a semiconductor device with a structure different from that of thesemiconductor device in FIG. 5A will be described.

For example, the transistors 103 and 104 can be provided in not only thesemiconductor device illustrated in FIG. 1A but also the semiconductordevices illustrated in FIGS. 3A to 3D and FIGS. 4A to 4C. When thesemiconductor devices illustrated in FIGS. 3A to 3D and FIGS. 4A to 4Care provided with the transistors 103 and 104, the semiconductor deviceshave a function and an advantageous effect that are similar to those ofthe semiconductor device in FIG. 5A.

FIG. 7A illustrates an example of the case where the transistors 103 and104 are provided in the semiconductor device in FIG. 3A.

FIG. 7B illustrates an example of the case where the transistors 103 and104 are provided in the semiconductor device in FIG. 3B.

FIG. 7C illustrates an example of the case where the transistors 103 and104 are provided in the semiconductor device in FIG. 3C. In thesemiconductor device in FIG. 7C, the wiring 113 can be omitted, so thatthe number of wirings and the number of signals can be reduced.

FIG. 8A illustrates an example of the case where the transistors 103 and104 are provided in the semiconductor device in FIG. 4A.

FIG. 8B illustrates an example of the case where the transistors 103 and104 are provided in the semiconductor device in FIG. 4B.

FIG. 8C illustrates an example of the case where the transistors 103 and104 are provided in the semiconductor device in FIG. 4C.

For example, the transistor 104 can be omitted in each of thesemiconductor devices illustrated in FIG. 5A, FIGS. 7A to 7C, and FIGS.8A to 8C. The number of transistors can be reduced by omitting thetransistor 104, so that the layout area can be decreased.

FIG. 9A illustrates an example of the case where the transistor 104 isomitted in the semiconductor device in FIG. 5A.

FIG. 9B illustrates an example of the case where the transistor 104 isomitted in the semiconductor device in FIG. 7C.

Next, an example of a circuit having a function of controlling asemiconductor device (such a circuit is also referred to as a controlcircuit) will be described.

FIG. 10 illustrates a circuit 130 for controlling a semiconductordevice. In FIG. 10, the semiconductor device illustrated in FIG. 5A isused as the semiconductor device; however, the semiconductor device tobe used is not limited to the semiconductor device in FIG. 5A. Forexample, any of the semiconductor devices in Embodiment 1, thisembodiment, or the other embodiments can be used as the semiconductordevice.

The circuit 130 has a function of applying a potential to each wiring ofthe semiconductor device. That is, the circuit 130 has a function ofcontrolling the timing of outputting a signal or supplying a voltage toeach wiring of the semiconductor device.

The circuit 130 includes a circuit 131, a circuit 132, a circuit 133,and a circuit 134. The circuit 131 has a function of supplying thevoltage V1 to the wiring 111 or a function of supplying a signal to thewiring 111. The circuit 132 has a function of supplying a signal to thewiring 113. The circuit 133 has a function of supplying a signal to thewiring 114. The circuit 134 has a function of supplying the voltage V1to the wiring 115. That is, each of the circuits 131, 132, and 133 has afunction of a signal generation circuit, a timing generator circuit, orthe like. Each of the circuits 131 and 134 has a function of a voltagegeneration circuit, a regulator circuit, or the like.

Note that each of the circuits 131 to 134 can be constituted by at leastone of an amplifier circuit, a bipolar transistor, a MOS transistor, acapacitor, a resistor, a coil, a DC voltage source, an AC voltagesource, a DC current source, and a switch.

Note that a protective circuit 140 can be connected to the wirings 113and 114. The protective circuit 140 includes a plurality of transistors141 and a plurality of transistors 142. A first terminal of thetransistor 141 is connected to the wiring 115. A second terminal of thetransistor 141 is connected to the wiring 113 or the wiring 114. A gateof the transistor 141 is connected to the wiring 115. A first terminalof the transistor 142 is connected to the wiring 111. A second terminalof the transistor 142 is connected to the wiring 113 or the wiring 114.A gate of the transistor 142 is connected to the wiring 113 or thewiring 114. Note that either the transistors 141 or the transistors 142can be omitted.

This embodiment can be combined with any of the other embodiments asappropriate.

Embodiment 3

In this embodiment, an example of a semiconductor device and an exampleof a method for driving the semiconductor device will be described. Inparticular, an example of a NOR circuit and a NAND circuit that includethe semiconductor device shown in Embodiment 2 and an example of amethod for driving the NOR circuit and the NAND circuit will bedescribed.

First, a structure with which the semiconductor device in Embodiment 2can have a function of a NOR circuit will be described.

In the semiconductor device in Embodiment 2, N transistors 103 (referredto as transistors 103_1 to 103_N, where N is a natural number) areconnected in parallel between the wiring 115 and the wiring 112. Gatesof the N transistors 103 are connected to N wirings 114 (wirings 114_1to 114_N), respectively. Moreover, N transistors 104 (referred to astransistors 104_1 to 104_N) are connected in parallel between the wiring115 and the node 11. Gates of the N transistors 104 are connected to therespective N wirings 114. For example, a first terminal of thetransistor 103 _(—) i (i is any one of 1 to N) is connected to thewiring 115. A second terminal of the transistor 103 _(—) i is connectedto the wiring 112. A gate of the transistor 103 _(—) i is connected tothe wiring 114 _(—) i. A first terminal of the transistor 104 _(—) i (iis any one of 1 to N) is connected to the wiring 115. A second terminalof the transistor 104 _(—) i is connected to the node 11. A gate of thetransistor 104 _(—) i is connected to the wiring 114 _(—) i. With such astructure, the semiconductor device in Embodiment 2 can have a functionof a NOR circuit with N inputs.

FIG. 11A illustrates an example of a NOR circuit obtained by adding theabove-described structure to the semiconductor device illustrated inFIG. 7C.

FIG. 11B illustrates an example of a NOR circuit obtained by adding theabove-described structure to the semiconductor device illustrated inFIG. 5A. In the NOR circuit in FIG. 11B, an inverted signal of a signalinput to any of the N wirings 114 may be input to the wiring 113.

Next, an example of operation of the NOR circuit including thesemiconductor device in Embodiment 2 will be described, using thesemiconductor device illustrated in FIG. 11A as an example. Here, thefollowing two operations will be described: the operation in the casewhere the potential V1 is applied to at least one of the N wirings 114,and the operation in the case where the potential V2 is applied to allthe N wirings 114.

First, the operation in the case where the potential V1 is applied to atleast one of the N wirings 114 will be described. It is assumed that thepotential V1 is applied to the wiring 114_1 and the potential V2 isapplied to the other wirings (the wirings 114_2 to 114_N), and that thepotential V1 is applied to the wiring 111 and the potential V2 isapplied to the wiring 115. Accordingly, the transistor 104_1 is turnedon and the transistors 104_2 to 104_N are turned off, so that electricalcontinuity is established between the wiring 115 and the node 11. Atthis time, the transistor 102 is turned on, so that electricalcontinuity is established between the wiring 111 and the node 11. Insuch a manner, the node 11 is supplied with the potential of the wiring115 and the potential of the wiring 111. Thus, the potential of the node11 exceeds the potential of the wiring 115 (the potential V2) andbecomes lower than the potential of the wiring 111 (the potential V1).Assuming that the potential of the node 11 becomes lower than V2+Vth101here, the transistor 101 is turned off, so that electrical continuity isnot established between the wiring 111 and the wiring 112. At this time,the transistor 103_1 is turned on and the transistors 103_2 to 103_N areturned off, so that electrical continuity is established between thewiring 115 and the wiring 112. The potential of the wiring 115 issupplied to the wiring 112 in such a manner, so that the potential ofthe wiring 112 becomes equal to the potential V2 (see FIG. 12A).

Then, the operation in the case where the potential V2 is applied to allthe N wirings 114 will be described. Assuming that the potential V1 isapplied to the wiring 111 and the potential V2 is applied to the wiring115, the transistors 104_1 to 104_N are turned off, so that electricalcontinuity is not established between the wiring 115 and the node 11. Atthis time, the transistor 102 is turned on, so that electricalcontinuity is established between the wiring 111 and the node 11. Then,the potential of the wiring 111 is supplied to the node 11, so that thepotential of the node 11 starts to rise. Then, the potential of the node11 rises to V2+Vth101. Thus, the transistor 101 is turned on, so thatelectrical continuity is established between the wiring 111 and thewiring 112. At this time, the transistors 103_1 to 103_N are off, sothat electrical continuity is not established between the wiring 115 andthe wiring 112. The potential of the wiring 111 is supplied to thewiring 112 in such a manner, so that the potential of the wiring 112starts to rise. After that, the potential of the node 11 rises toV1−Vth102. Thus, the transistor 102 is turned off, so that electricalcontinuity between the wiring 111 and the node 11 is broken. Then, thenode 11 enters a floating state. At this time, the potential of thewiring 112 continues to rise. For that reason, the potential of the node11 rises to V1+Vth101+Va because of parasitic capacitance between thegate and the second terminal of the transistor 101. This is so-calledbootstrap operation. Accordingly, the potential of the wiring 112 risesto a value equal to that of the potential V1 (see FIG. 12B).

Note that the N transistors 103 preferably have the same channel width.If restriction of circuit layout or the like makes it difficult for theN transistors 103 to have the same channel width, it is preferable thatat least two of the N transistors 103 have the same channel width. Thisis because when the transistors 103 have the same channel width, thecircuit can be designed more easily and an operation malfunction can besuppressed. The same can be said for the transistors 104.

Note that the N transistors 103 preferably have high drive capability inorder to drive the wiring 112. Therefore, the channel width of at leastone of the N transistors 103 is preferably larger than that of at leastone of the N transistors 104. Specifically, the channel width of atleast one of the N transistors 103 is preferably larger than that of atleast one of the N transistors 104 and is preferably 10 times or less aslarge as that of at least one of the N transistors 104. The channelwidth of at least one of the N transistors 103 is more preferably 1.5 to7 times, further preferably 2 to 5 times as large as that of at leastone of the N transistors 104.

Note that in the case where the potential V2 is applied to the N wirings114, the potential of the node 11 preferably has a value with which thetransistor 101 is turned off. For that reason, the channel width of atleast one of the N transistors 104 is preferably larger than that of thetransistor 102. Specifically, the channel width of at least one of the Ntransistors 104 is preferably larger than that of the transistor 102 andis preferably 10 times or less as large as that of the transistor 102.The channel width of at least one of the N transistors 104 is morepreferably 2 to 5 times, further preferably 2.5 to 3.5 times as large asthat of the transistor 102.

Next, a structure with which the semiconductor device in Embodiment 2can have a function of a NAND circuit will be described.

The following is a structure with which the semiconductor device inEmbodiment 2 can have a function of a NAND circuit. In the semiconductordevice in Embodiment 2, the N transistors 103 (the transistors 103_1 to103_N, where N is a natural number) are connected in series between thewiring 115 and the wiring 112. Gates of the N transistors 103 areconnected to the N wirings 114 (the wirings 114_1 to 114_N),respectively. Moreover, the N transistors 104 (the transistors 104_1 to104_N) are connected in series between the wiring 115 and the node 11.Gates of the N transistors 104 are connected to the respective N wirings114. For example, a first terminal of the transistor 103 _(—) i (i isany one of 1 to N) is connected to a second terminal of the transistor103 _(—) i+1. A second terminal of the transistor 103 _(—) i isconnected to a first terminal of the transistor 103 _(—) i−1. A gate ofthe transistor 103 _(—) i is connected to the wiring 114 _(—) i. A firstterminal of the transistor 104 _(—) i (i is any one of 1 to N) isconnected to a second terminal of the transistor 104 _(—) i+1. A secondterminal of the transistor 104 _(—) i is connected to a first terminalof the transistor 104 _(—) i−1. A gate of the transistor 104 _(—) i isconnected to the wiring 114 _(—) i. Note that a second terminal of thetransistor 103_1 is connected to the wiring 112; a first terminal of thetransistor 103_N is connected to the wiring 115; a second terminal ofthe transistor 104_1 is connected to the node 11; and a first terminalof the transistor 104_N is connected to the wiring 115. With such astructure, the semiconductor device in Embodiment 2 can have a functionof a NAND circuit with N inputs.

FIG. 13A illustrates an example of a NAND circuit obtained by adding theabove-described structure to the semiconductor device illustrated inFIG. 7C.

FIG. 13B illustrates an example of a NAND circuit obtained by adding theabove-described structure to the semiconductor device illustrated inFIG. 5A. Note that an inverted signal of a signal input to any of the Nwirings 114 is input to the wiring 113.

Next, an example of operation of the NAND circuit including thesemiconductor device in Embodiment 2 will be described, using thesemiconductor device illustrated in FIG. 13A as an example. Here, thefollowing two operations will be described: the operation in the casewhere the potential V2 is applied to at least one of the N wirings 114,and the operation in the case where the potential V1 is applied to allthe N wirings 114.

First, the operation in the case where the potential V2 is applied to atleast one of the N wirings 114 will be described. It is assumed that thepotential V1 is applied to the wiring 114_1 and the potential V2 isapplied to the other wirings (the wirings 114_2 to 114_N), and that thepotential V1 is applied to the wiring 111 and the potential V2 isapplied to the wiring 115. Accordingly, the transistor 104_1 is turnedon and the transistors 104_2 to 104_N are turned off, so that electricalcontinuity is not established between the wiring 115 and the node 11. Atthis time, the transistor 102 is turned on, so that electricalcontinuity is established between the wiring 111 and the node 11. Then,the potential of the wiring 111 is supplied to the node 11, so that thepotential of the node 11 starts to rise. Then, the potential of the node11 rises to V2+Vth101. Thus, the transistor 101 is turned on, so thatelectrical continuity is established between the wiring 111 and thewiring 112. At this time, the transistor 103_1 is turned on and thetransistors 103_2 to 103_N are turned off, so that electrical continuityis not established between the wiring 115 and the wiring 112. Thepotential of the wiring 111 is supplied to the wiring 112 in such amanner, so that the potential of the wiring 112 starts to rise. Afterthat, the potential of the node 11 rises to V1−Vth102. Thus, thetransistor 102 is turned off, so that electrical continuity between thewiring 111 and the node 11 is broken. Then, the node 11 enters afloating state. At this time, the potential of the wiring 112 continuesto rise. For that reason, the potential of the node 11 rises toV1+Vth101+Va due to parasitic capacitance between the gate and thesecond terminal of the transistor 101. This is so-called bootstrapoperation. Accordingly, the potential of the wiring 112 rises to a valueequal to that of the potential V1 (see FIG. 14A).

Then, the operation in the case where the potential V1 is applied to allthe N wirings 114 will be described. Assuming that the potential V1 isapplied to the wiring 111 and the potential V2 is applied to the wiring115, the transistors 104_1 to 104_N are turned on, so that electricalcontinuity is established between the wiring 115 and the node 11. Atthis time, the transistor 102 is turned on, so that electricalcontinuity is established between the wiring 111 and the node 11. Insuch a manner, the node 11 is supplied with the potential of the wiring115 and the potential of the wiring 111. Thus, the potential of the node11 exceeds the potential of the wiring 115 (the potential V2) andbecomes lower than the potential of the wiring 111 (the potential V1).Assuming that the potential of the node 11 is lower than V2+Vth102 here,the transistor 102 is turned off, so that electrical continuity is notestablished between the wiring 111 and the wiring 112. At this time, thetransistors 103_1 to 103_N are turned on, so that electrical continuityis established between the wiring 115 and the wiring 112. The potentialof the wiring 115 is supplied to the wiring 112 in such a manner, sothat the potential of the wiring 112 becomes equal to the potential V2(see FIG. 14B).

Note that the N transistors 103 preferably have the same channel width.If restriction of circuit layout or the like makes it difficult for theN transistors 103 to have the same channel width, it is preferable thatat least two of the N transistors 103 have the same channel width. Thisis because when the transistors 103 have the same channel width, thecircuit can be designed more easily and an operation malfunction can besuppressed. The same can be said for the transistors 104.

Note that the channel width of the N transistors 103 is preferably largein order to shorten the fall time of the potential of the wiring 112.However, the layout area is increased if the channel width is too large.For that reason, at least one channel width of the N transistors 103 ispreferably N times or less as large as the channel width of thetransistor 101. At least one channel width of the N transistors 103 ismore preferably ⅓ to 3 times, further preferably ½ to 2 times as largeas the channel width of the transistor 101.

Note that in the case where the potential V1 is applied to all the Nwirings 114, the channel width of the N transistors 104 is preferablylarge in order that the potential of the node 11 is lower thanV2+Vth101. However, the layout area is increased if the channel width istoo large. For that reason, the channel width of at least one of the Ntransistors 104 is preferably N times or less as large as that of thetransistor 102. The width of at least one of the N transistors 104 ismore preferably ⅓ to 3 times, further preferably ½ to 2 times as largeas that of the transistor 102.

As described above, the NOR circuit and the NAND circuit in thisembodiment can be constituted using the semiconductor device shown inEmbodiment 2. Thus, the NOR circuit and the NAND circuit in thisembodiment can obtain advantageous effects similar to those of thesemiconductor devices in Embodiments 1 and 2.

This embodiment can be combined with any of the other embodiments asappropriate.

Embodiment 4

In this embodiment, an example of a semiconductor device and an exampleof a method for driving the semiconductor device will be described. Inparticular, an example of a decoder circuit including the semiconductordevice shown in Embodiment 3 and an example of a method for driving thedecoder circuit will be described.

First, an example of a structure of a semiconductor device in thisembodiment will be described.

FIG. 16 illustrates an example of a decoder circuit in this embodiment.The decoder circuit in FIG. 16 includes m NOR circuits 201 (referred toas NOR circuits 201_1 to 201 _(—) m, where m is a natural number).

Note that any of the NOR circuits shown in Embodiment 3 is preferablyused as the m NOR circuits 201.

A control signal of N bits (N is a natural number, where 2^(N)>m) isinput to each of the m NOR circuits 201. The control signal of N bits isselected from control signals D1 to DN and control signals Db1 to DbN.The control signals Db1 to DbN are inverted signals of the controlsignals D1 to DN. Control signals input to the m NOR circuits 201 aredifferent from each other. For example, the control signal D1 to DN areinput to the NOR circuit 201_1. The control signal Db1 and the controlsignals D2 to DN are input to the NOR circuit 2012. The control signalD1, the control signal Db2, and the control signals D3 to DN are inputto the NOR circuit 201_3. The control signals input to the m NORcircuits 201 are made different from each other in such a manner, sothat only a signal output from any one of the m NOR circuits 201 canhave a different value from signals output from the other NOR circuits201. Specifically, the signal output from any one of the m NOR circuits201 can be an H-level signal and the signals output from the other NORcircuits 201 can be L-level signals. Moreover, when values of thecontrol signals D1 to DN and the control signals Db1 to DbN are changedevery predetermined period (e.g., per one gate selection period), theNOR circuits 201_1 to 201 _(—) m can sequentially output an H-levelsignal. Alternatively, the m NOR circuits 201 can output an H-levelsignal in given order.

Note that the control signals D1 to DN are input to the decoder circuitthrough N wirings 212 (referred to as wirings 212_1 to 212_N). Thecontrol signals Db1 to DbN are input to the decoder circuit through Nwirings 213 (referred to as wirings 213_1 to 213_N). Output signals ofthe m NOR circuits 201 are output to respective m wirings 211 (wirings211_1 to 211 _(—) m).

Note that the control signals Db1 to DbN can be generated by invertingthe control signals D1 to DN by an inverter circuit or the like. As theinverter circuit used for generating the control signals Db1 to DbN, anyof the semiconductor devices shown in Embodiment 1 can be used, forexample.

A decoder circuit can be constituted not only by NOR circuits but alsoby NAND circuits. As the NAND circuit, any of the NAND circuits shown inEmbodiment 3 is preferably used. FIG. 17 is a circuit diagram of adecoder circuit constituted by NAND circuits. The decoder circuit inFIG. 17 differs from the decoder circuit in FIG. 16 in that m NANDcircuits 202 (referred to as NAND circuits 202_1 to 202 _(—) m) are usedinstead of m NOR circuits 201.

In the decoder circuit in FIG. 17, a signal output from any one of the mNAND circuits 202 is an L-level signal and signals output from the otherNAND circuits 202 are H-level signals. For that reason, m invertercircuits 203 (referred to as inverter circuits 203_1 to 203 _(—) m) canbe provided when needed. Output signals of the m NAND circuits 202 areoutput to the m wirings 211 through the m inverter circuits 203.

As described above, the decoder circuit in this embodiment can beconstituted by the NOR circuit or the NAND circuit shown in Embodiment3. Thus, the decoder circuit in this embodiment can obtain advantageouseffects similar to those of the semiconductor devices in Embodiments 1and 2.

This embodiment can be combined with any of the other embodiments asappropriate.

Embodiment 5

In this embodiment, an example of a structure of a semiconductor deviceand an example of a process for manufacturing the semiconductor devicewill be described. In particular, an example of a thin film transistorin which a channel formation region is formed using an oxidesemiconductor and an example of a process for manufacturing the thinfilm transistor will be described.

<Example of Structure of Transistor>

FIG. 15D is a cross-sectional view of a transistor 450 (e.g., a thinfilm transistor) which is an example of a semiconductor device. Thetransistor 450 in FIG. 15D is an inverted staggered thin filmtransistor. Although FIG. 15D illustrates a single-gate thin filmtransistor, a multi-gate thin film transistor including a plurality ofchannel formation regions may be used as needed. The thin filmtransistor is an n-channel transistor in the following description;alternatively, a p-channel transistor may be used.

The transistor 450 includes a gate electrode layer 411 provided over asubstrate 400, a gate insulating layer 402 that covers the gateelectrode layer 411, an oxide semiconductor layer 406 a provided overthe gate electrode layer 411, and source/drain electrode layers 408 aand 408 b electrically connected to the oxide semiconductor layer 406 a.Moreover, an insulating layer 412 and an insulating layer 418 areprovided over the transistor 450. Note that the insulating layer 412 andthe insulating layer 418 are not essential and thus can be omitted asappropriate.

For the oxide semiconductor layer 406 a, a four-component metal oxidesuch as an In—Sn—Ga—Zn—O-based metal oxide; a three-component metaloxide such as an In—Ga—Zn—O-based metal oxide, an In—Sn—Zn—O-based metaloxide, an In—Al—Zn—O based metal oxide, a Sn—Ga—Zn—O-based metal oxide,an Al—Ga—Zn—O-based metal oxide, or a Sn—Al—Zn—O-based metal oxide; atwo-component metal oxide such as an In—Zn—O-based metal oxide, aSn—Zn—O-based metal oxide, an Al—Zn—O-based metal oxide, a Zn—Mg—O-basedmetal oxide, a Sn—Mg—O-based metal oxide, or an In—Mg—O-based metaloxide; an In—O-based metal oxide, a Sn—O-based metal oxide, a Zn—O-basedmetal oxide, or the like is used.

In particular, an In—Ga—Zn—O-based oxide semiconductor material ispreferably employed as a semiconductor material used for a semiconductordevice because it has sufficiently high resistance when no electricfield is applied and thus can realize a sufficiently small off-statecurrent, and because it has high field-effect mobility.

A typical example of the In—Ga—Zn—O-based oxide semiconductor materialis an oxide semiconductor material represented by InGaO₃(ZnO), (m islarger than 0 and is not a natural number). Moreover, there is an oxidesemiconductor material represented by 1 nMO₃(ZnO)_(m) (m is larger than0 and is not a natural number), using M instead of Ga. Here, M denotesone or more of metal elements selected from gallium (Ga), aluminum (Al),iron (Fe), nickel (Ni), manganese (Mn), cobalt (Co), and the like. Forexample, M can be Ga and Al, Ga and Fe, Ga and Ni, Ga and Mn, or Ga andCo. Note that the above composition is derived from a crystal structureand thus is just an example. In addition, an oxide semiconductormaterial expressed by In—Ga—Zn—O in this specification isInGaO₃(ZnO)_(m) (m is larger than 0 and is not a natural number), and itcan be confirmed using analysis with ICP-MS or RBS that m is not anatural number.

The hydrogen concentration of the oxide semiconductor layer ispreferably 5×10¹⁹ (atoms/cm³) or less.

<Method for Manufacturing Transistor>

Next, a method for manufacturing the above thin film transistor isdescribed with reference to FIGS. 15A to 15D.

First, the gate electrode layer 411 is formed over the substrate 400,and then, the gate insulating layer 402 is formed so as to cover thegate electrode layer 411. After that, an oxide semiconductor layer 406is formed over the gate insulating layer 402 (see FIG. 15A).

As the substrate 400, a glass substrate can be used, for example. Theglass substrate is preferably a non-alkali glass substrate. For thenon-alkali glass substrate, a glass material such as aluminosilicateglass, aluminoborosilicate glass, or barium borosilicate glass is used,for example. Instead of a glass substrate, the substrate 400 can be aninsulating substrate formed using an insulator, such as a ceramicsubstrate, a quartz substrate, or a sapphire substrate; a semiconductorsubstrate that is formed using a semiconductor material such as siliconand has a surface covered with an insulating material; or a conductivesubstrate that is formed using a conductive material such as metal orstainless steel and has a surface covered with an insulating material.Furthermore, a substrate formed from a flexible synthetic resin such asplastic generally tends to have a low upper temperature limit, but canbe used as the substrate 400 as long as the substrate can withstandprocessing temperatures in the subsequent manufacturing steps.

The gate electrode layer 411 can be formed in such a manner that aconductive layer is formed over the substrate 400 and selectivelyetched. The gate electrode layer 411 can be formed by a physical vapordeposition (PVD) method such as a sputtering method or a chemical vapordeposition (CVD) method such as a plasma CVD method. Moreover, the gateelectrode layer 411 can be formed using a metal material selected fromaluminum, chromium, copper, tantalum, titanium, molybdenum, andtungsten; an alloy containing any of these elements; or the like. Amaterial containing one or more of manganese, magnesium, zirconium,and/or beryllium may be used. A material that contains aluminum and oneor more of elements selected from titanium, tantalum, tungsten,molybdenum, chromium, neodymium, and scandium may be used.

Alternatively, the gate electrode layer 411 may be formed using aconductive metal oxide. As the conductive metal oxide, indium oxide(In₂O₃), tin oxide (SnO₂), zinc oxide (ZnO), an alloy of indium oxideand tin oxide (In₂O₃—SnO₂, sometimes referred to as ITO), an alloy ofindium oxide and zinc oxide (In₂O₃—ZnO), or any of the metal oxidematerials containing silicon or silicon oxide can be used.

The gate electrode layer 411 may have a single-layer structure or alayered structure of two layers or more. Note that in this embodiment,heat treatment at relatively high temperature is performed after theformation of the gate electrode layer 411; therefore, the gate electrodelayer 411 is preferably formed using a material with heat resistancehigh enough to withstand the heat treatment. Examples of the materialwith heat resistance are titanium, tantalum, tungsten, and molybdenum.Moreover, it is possible to use polysilicon whose conductivity isincreased by addition of an impurity element.

The gate insulating layer 402 can be formed by a CVD method, asputtering method, or the like. The gate insulating layer 402 ispreferably formed using silicon oxide, silicon nitride, siliconoxynitride, silicon nitride oxide, aluminum oxide, hafnium oxide,tantalum oxide, or the like. The gate insulating layer 402 may have asingle-layer structure or a layered structure of two layers or more. Thegate insulating layer 402 can have a thickness of 10 nm to 500 nm, forexample.

When the gate insulating layer 402 is formed using a high-k materialsuch as hafnium silicate (HfSiO_(x)), hafnium silicate to which nitrogenis added (HfSi_(x)O_(y)N_(z)), hafnium aluminate to which nitrogen isadded (HfAl_(x)O_(y)N_(z)), hafnium oxide, or yttrium oxide, gateleakage can be reduced. Further, the gate insulating layer 402 can havea layered structure in which a layer including a high-k material and oneor more of a silicon oxide layer, a silicon nitride layer, a siliconoxynitride layer, a silicon nitride oxide layer, and an aluminum oxidelayer are stacked.

Note that the gate insulating layer 402 is preferably formed so as tocontain impurities such as hydrogen and water as little as possible.This is because if hydrogen and water are contained in the gateinsulating layer 402, hydrogen might enter the oxide semiconductor layer406 a and extract oxygen in the oxide semiconductor layer 406 a, whichmight lead to deterioration of characteristics of the transistor.

For example, the gate insulating layer 402 is formed by a sputteringmethod or the like, the gate insulating layer 402 is preferably formedin a state where moisture remaining in a treatment chamber is removed.An entrapment vacuum pump is preferably used in order to remove moistureremaining in a treatment chamber. For example, a cryopump, an ion pump,or a titanium sublimation pump can be used. A turbo pump provided with acold trap may be used. In a treatment chamber that is evacuated with acryopump or the like, hydrogen, water, and the like are sufficientlyremoved, so that the concentration of impurities contained in the gateinsulating layer 402 can be reduced.

In addition, it is preferable to employ a high-density plasma CVD methodusing a microwave (e.g., a frequency of 2.45 GHz) because the gateinsulating layer 402 can be dense and have high withstand voltage andhigh quality. When the oxide semiconductor layer 406 a and thehigh-quality gate insulating layer 402 are in close contact with eachother, the interface state density can be reduced and interfaceproperties can be favorable. In particular, it is preferable to use ahigh-density plasma apparatus with which a plasma density of 1×10¹¹/cm³or higher can be achieved. Properties of the interface between the gateinsulating layer 402 and the oxide semiconductor layer 406 a can be madefavorable and impurities of the oxide semiconductor, particularlyhydrogen and water, are reduced as described above; thus, it is possibleto obtain a stable transistor whose threshold voltage (Vth) does notchange through a gate bias-temperature stress test (BT test; forexample, at 85° C. with 2×10⁶ V/cm for 12 hours).

In forming the gate electrode layer 411, it is preferable to use ahigh-purity gas from which impurities such as hydrogen and water isreduced to a concentration of approximately a few parts per million(ppm) or a few parts per billion (ppb).

Note that the oxide semiconductor layer that becomes an i-type oxidesemiconductor layer or a substantially i-type oxide semiconductor layer(a highly purified oxide semiconductor layer) in a later step isextremely sensitive to the interface state density or interface electriccharge; therefore, the interface with the gate insulating layer isimportant. For that reason, the gate insulating layer (GI) which is incontact with the highly purified oxide semiconductor layer needs to havehigh quality. Therefore, high-density plasma CVD with use of microwaves(2.45 GHz) is preferably employed because a dense and high-qualityinsulating film having high withstand voltage can be formed. The highlypurified oxide semiconductor and the high-quality gate insulating layerare in close contact with each other, so that the interface statedensity can be reduced and favorable interface characteristics can beobtained. It is important that the insulating layer has favorablequality as the gate insulating layer and can reduce the interface statedensity with the oxide semiconductor layer so that a favorable interfacecan be formed.

The oxide semiconductor layer 406 can be formed by a sputtering methodin a rare gas (typically argon) atmosphere, an oxygen atmosphere, or anatmosphere including a rare gas (typically argon) and oxygen. As anatmosphere in which the oxide semiconductor layer 406 is formed, it ispreferable to use a high-purity gas atmosphere, for example, from whichimpurities such as hydrogen, water, a hydroxyl group, and hydride areremoved to a concentration of a few parts per million (preferably, a fewparts per billion).

Before the oxide semiconductor layer 406 is deposited by a sputteringmethod, powdery substances (also referred to as particles or dust)attached on a surface of the gate insulating layer 402 are preferablyremoved by reverse sputtering in which an argon gas is introduced andplasma is generated. The reverse sputtering refers to a method in which,without application of voltage to a target side, an RF power source isused for application of voltage in order to generate plasma in thevicinity of the substrate to modify a surface. Note that instead of anargon atmosphere, nitrogen, helium, oxygen, or the like may be used.

The oxide semiconductor layer 406 can be formed using a four-componentmetal oxide such as an In—Sn—Ga—Zn—O-based metal oxide; athree-component metal oxide such as an In—Ga—Zn—O-based metal oxide, anIn—Sn—Zn—O-based metal oxide, an In—Al—Zn—O based metal oxide, aSn—Ga—Zn—O-based metal oxide, an Al—Ga—Zn—O-based metal oxide, or aSn—Al—Zn—O-based metal oxide; a two-component metal oxide such as anIn—Zn—O-based metal oxide, a Sn—Zn—O-based metal oxide, an Al—Zn—O-basedmetal oxide, a Zn—Mg—O-based metal oxide, a Sn—Mg—O-based metal oxide,or an In—Mg—O-based metal oxide; an In—O-based metal oxide, a Sn—O-basedmetal oxide, a Zn—O-based metal oxide, or the like.

In particular, an In—Ga—Zn—O-based oxide semiconductor material ispreferably employed as a semiconductor material used for a semiconductordevice because it has sufficiently high resistance when no electricfield is applied and can realize a sufficiently small off-state current,and because it has high field-effect mobility.

In this embodiment, the amorphous oxide semiconductor layer 406 isformed as the oxide semiconductor layer 406 by a sputtering method withthe use of an In—Ga—Zn—O-based oxide semiconductor target.

As a target used for forming the In—Ga—Zn—O-based oxide semiconductorlayer 406 by a sputtering method, a target represented by a compositionratio of In:Ga:Zn=1:x:y (x is 0 or larger and y is 0.5 to 5) may beused. For example, a target with a composition ratio of In:Ga:Zn=1:1:1[atomic ratio] (x=1 and y=1; that is, In₂O₃:Ga₂O₃:ZnO=1:1:2 [molarratio]) may be used. As the oxide semiconductor target, it is possibleto use a target with a composition ratio of In:Ga:Zn=1:1:0.5 [atomicratio], a target with a composition ratio of In:Ga:Zn=1:1:2 [atomicratio], or a target with a composition ratio of In:Ga:Zn=1:0:1 [atomicratio] (x=0 and y=1). Moreover, the oxide semiconductor layer 406 can bedeposited using a target containing SiO₂ at 2 wt % to 10 wt % so as tocontain SiO_(x) (X>0).

The relative density of the oxide semiconductor in the oxidesemiconductor target is 80% or more, preferably 95% or more, furtherpreferably 99.9% or more. By using an oxide semiconductor with a highrelative density, the dense oxide semiconductor layer 406 can be formed.

In forming the oxide semiconductor layer 406, for example, the substrateis held in a treatment chamber that is maintained at reduced pressure,and the substrate is heated to 100° C. to 600° C., preferably 200° C. to400° C. Then, a sputtering gas from which hydrogen and water are removedis introduced into the treatment chamber from which remaining moistureis being removed, and the oxide semiconductor layer 406 is formed usingthe above-described target. The oxide semiconductor layer 406 is formedwhile the substrate is heated, so that impurities contained in the oxidesemiconductor layer 406 can be reduced. In addition, damage bysputtering can be reduced. In order to remove moisture remaining in thetreatment chamber, the above-described entrapment vacuum pump ispreferably used. Hydrogen, water, and the like are removed in thetreatment chamber that is evacuated with a cryopump, for example; thus,the concentration of impurities contained in the oxide semiconductorlayer 406 can be reduced.

For example, the conditions for forming the oxide semiconductor layer406 can be set as follows: the distance between the substrate and thetarget is 170 mm; the pressure is 0.4 Pa; the direct-current (DC) poweris 0.5 kW; and the atmosphere is an oxygen atmosphere (the flow rateratio of oxygen is 100%), an argon atmosphere (the flow rate ratio ofargon is 100%), or an atmosphere including oxygen and argon. Note that apulse direct current (DC) power supply is preferable because dust(powder or flake-like substances formed at the time of the filmformation) can be reduced and the film thickness can be uniform. Thethickness of the oxide semiconductor layer 406 is 2 nm to 200 nm,preferably 5 nm to 30 nm Note that since an appropriate thickness of theoxide semiconductor layer 406 differs depending on an oxidesemiconductor material to be used, application, and the like, thethickness is set in accordance with the material to be used,application, and the like.

Next, the oxide semiconductor layer 406 is selectively etched to formthe island-shaped oxide semiconductor layer 406 a. After that, aconductive layer is formed so as to cover the gate insulating layer 402and the oxide semiconductor layer 406 a and is etched, so that thesource/drain electrode layers 408 a and 408 b are formed (see FIG. 15B).

The oxide semiconductor layer can be etched by dry etching, wet etching,or a combination of dry etching and wet etching. The etching conditions(e.g., an etching gas or an etching solution, etching time, andtemperature) are set as appropriate depending on the material so thatthe oxide semiconductor layer can be etched into a desired shape.

As dry etching, a parallel plate RIE (reactive ion etching) method, anICP (inductively coupled plasma) etching method, or the like can beused. Also in this case, the etching conditions (e.g., the amount ofelectric power applied to a coiled electrode, the amount of electricpower applied to an electrode on the substrate side, and the electrodetemperature on the substrate side) need to be set as appropriate.

An example of an etching gas that can be used for dry etching is a gascontaining chlorine (a chlorine-based gas such as chlorine (Cl₂), boronchloride (BCl₃), silicon chloride (SiCl₄), or carbon tetrachloride(CCl₄)). Moreover, a gas containing fluorine (a fluorine-based gas suchas carbon tetrafluoride (CF₄), sulfur hexafluoride (SF₆), nitrogentrifluoride (NF₃), or trifluoromethane (CHF₃)), hydrogen bromide (HBr),oxygen (O₂), any of these gases to which a rare gas such as helium (He)or argon (Ar) is added, or the like may be used.

Examples of an etchant that can be used for wet etching are a solutionobtained by mixing phosphoric acid, acetic acid, and nitric acid; and anammonia peroxide mixture (hydrogen peroxide water at 31 wt %: ammoniawater at 28 wt %: water=5:2:2). An etchant such as ITO07N (produced byKanto Chemical Co., Inc.) may also be used.

Next, first heat treatment is preferably performed on the oxidesemiconductor layer 406 a. An excess amount of water (including ahydroxyl group), hydrogen, and the like that are contained in the oxidesemiconductor layer 406 a can be removed by the first heat treatment.The temperature of the first heat treatment is, for example, higher thanor equal to 400° C. and lower than or equal to 750° C., or higher thanor equal to 400° C. and less than the strain point of the substrate. Thefirst heat treatment can be performed, for example, at 450° C. for onehour in a nitrogen atmosphere after the substrate 400 is introduced intoan electric furnace including a resistance heating element. During thefirst heat treatment, it is preferable that the oxide semiconductorlayer 406 a be not exposed to the air in order to prevent entry of waterand hydrogen.

The heat treatment apparatus is not limited to an electric furnace; theheat treatment apparatus can be an apparatus that heats an object withthermal conduction or thermal radiation given by a medium such as aheated gas. For example, an RTA apparatus employing a lamp heatingmethod (a lamp rapid thermal anneal (LRTA) apparatus), an RTA apparatusemploying a gas heating method using a heated gas (a gas rapid thermalanneal (GRTA) apparatus), an RTA apparatus employing both a lamp heatingmethod and a gas heating method, or the like can be used. In the casewhere an apparatus employing a gas heating method is used, an inert gasthat does not react with an object to be processed by heat treatment,for example, nitrogen or a rare gas such as argon is used.

For example, as the first heat treatment, a GRTA process may beperformed as follows. The substrate is put in an inert gas atmospherethat has been heated to a high temperature of 650° C. to 700° C., heatedfor several minutes, and taken out of the inert gas atmosphere. The GRTAprocess enables high-temperature heat treatment for a short time.Moreover, the GRTA process can be employed even when the temperatureexceeds the upper temperature limit of the substrate because it is heattreatment for a short time. For example, in the case where a glasssubstrate is used, shrinkage of the substrate becomes a problem attemperatures higher than the upper temperature limit (the strain point)of the substrate; however, shrinkage is not a problem in heat treatmentfor a short time. Note that an inert gas may be switched to a gasincluding oxygen during the treatment. This is because defects due tooxygen vacancy can be reduced by performing the first heat treatment inan atmosphere including oxygen.

Note that the inert gas atmosphere is preferably an atmosphere thatcontains nitrogen or a rare gas (e.g., helium, neon, or argon) as itsmain component and does not contain water, hydrogen, or the like. Forexample, the purity of nitrogen or a rare gas (e.g., helium, neon, orargon) introduced into a heat treatment apparatus is 6N (99.9999%) orhigher, preferably 7N (99.99999%) or higher (i.e., the concentration ofthe impurities is less than or equal to 1 ppm, preferably less than orequal to 0.1 ppm). The first heat treatment may be performed inultra-dry air with an H₂O concentration of 20 ppm or lower, preferablyin ultra-dry air with an H₂O concentration of 1 ppm or lower. By suchfirst heat treatment, water (including a hydroxyl group), hydrogen, andthe like that are contained in the oxide semiconductor layer 406 can beremoved.

By performing the above-described first heat treatment, hydrogencontained in the oxide semiconductor layer 406 can be reduced(preferably removed), so that the oxide semiconductor layer 406 can behighly purified so as to contain an impurity other than its maincomponent as little as possible.

Note that the first heat treatment performed on the oxide semiconductorlayer can also be performed on the oxide semiconductor layer 406 whichhas not yet been processed into the island-shaped oxide semiconductorlayer. In that case, after the first heat treatment, the substrate istaken out from a heating apparatus, and then, etching is performed usinga mask, for example.

The heat treatment for dehydration or dehydrogenation of the oxidesemiconductor layer may be performed at any of the following timings:after the oxide semiconductor layer is formed; after the sourceelectrode layer and the drain electrode layer are stacked over the oxidesemiconductor layer; and after a protective insulating film is formedover the source electrode layer and the drain electrode layer.

The source/drain electrode layers 408 a and 408 b are formed in thefollowing manner: a conductive layer is formed so as to cover the oxidesemiconductor layer 406 a and then the conductive layer is selectivelyetched. The conductive layer can be formed by a sputtering method or avacuum evaporation method. The conductive layer can be formed using ametal material selected from aluminum, chromium, copper, tantalum,titanium, molybdenum, and tungsten; an alloy material including any ofthese elements; an alloy material including the above elements incombination; or the like. One or more materials selected from manganese,magnesium, zirconium, beryllium, and yttrium may be used. A materialthat contains aluminum and one or more of elements selected fromtitanium, tantalum, tungsten, molybdenum, chromium, neodymium, andscandium may be used.

The source/drain electrode layers 408 a and 408 b can have asingle-layer structure or a layered structure of two layers or more andcan have, for example, a single-layer structure of an aluminum filmcontaining silicon; a two-layer structure in which a titanium film isstacked over an aluminum film; or a three-layer structure in which atitanium film, an aluminum film, and a titanium film are stacked in theorder.

In the case where heat treatment for dehydration or dehydrogenation ofthe oxide semiconductor layer 406 a is performed on the conductivelayer, it is preferable to use a conductive layer with heat resistancehigh enough to withstand the heat treatment.

Materials and etching conditions are adjusted as appropriate so that theoxide semiconductor layer 406 a is not removed when the conductive layeris etched.

In this embodiment, a titanium film is used as the conductive layer; anIn—Ga—Zn—O-based oxide is used for the oxide semiconductor layer 406 a;and an ammonia hydrogen peroxide solution (a mixed solution of ammonia,water, and a hydrogen peroxide solution) is used as an etchant.

Note that in etching of the conductive layer, only part of the oxidesemiconductor layer 406 a is etched and the oxide semiconductor layer406 a having a groove (a recessed portion) is formed in some cases.Moreover, a mask used in the etching step may be formed by an inkjetmethod. A photomask is not used when the mask is formed by an inkjetmethod, which results in reducing manufacturing costs.

In order to reduce the number of photomasks used in a photolithographystep and reduce the number of steps, an etching step may be performedwith the use of a multi-tone mask which is a light-exposure mask throughwhich light is transmitted to have a plurality of intensities. Since aresist mask formed using a multi-tone mask has a plurality ofthicknesses and can be further changed in shape by ashing, the resistmask can be used in a plurality of etching steps for providing differentpatterns. Therefore, a resist mask corresponding to at least two kindsof different patterns can be formed by one multi-tone mask. Thus, thenumber of light-exposure masks can be reduced and the number ofcorresponding photolithography steps can be also reduced, whereby theprocess can be simplified.

Next, plasma treatment using a gas such as nitrous oxide (N₂O), nitrogen(N₂), or argon (Ar) is performed. By this plasma treatment, absorbedwater and the like attached to an exposed surface of the oxidesemiconductor layer are removed. Plasma treatment may be performed usinga mixture gas of oxygen and argon.

Then, the insulating layer 412 is formed so as to cover the oxidesemiconductor layer 406 a and the source/drain electrode layers 408 aand 408 b (see FIG. 15C).

The insulating layer 412 can be formed by a sputtering method, a CVDmethod, or the like, that is, a method with which impurities such asmoisture and hydrogen are not mixed into the insulating layer 412. Ifhydrogen is contained in the insulating layer 412, hydrogen enters theoxide semiconductor layer 406 a so that the backchannel of the oxidesemiconductor layer 406 a is made to have lower resistance (to be ann-type layer); thus, a parasitic channel is formed. Therefore, it isimportant to employ a formation method in which hydrogen is not used sothat the insulating layer 412 contains as little hydrogen as possible.

The insulating layer 412 is preferably formed using silicon oxide,silicon oxynitride, aluminum oxide, hafnium oxide, tantalum oxide, orthe like. In particular, a silicon oxide film formed by a sputteringmethod is preferably used. Note that the insulating layer 412 can have asingle-layer structure or a layered structure. Although not particularlylimited, the insulating layer 412 can have a thickness of, for example,10 nm to 500 nm, preferably 50 nm to 200 nm.

Next, second heat treatment is preferably performed on the oxidesemiconductor layer 406 a in an inert gas atmosphere or an oxygenatmosphere. By performing the second heat treatment, oxygen can besupplied to oxygen vacancy of the oxide semiconductor layer 406 a, andthe intrinsic (i-type) or substantially intrinsic oxide semiconductorlayer can be formed. The second heat treatment can reduce variations inelectric characteristics of transistors. The second heat treatment isperformed at 200° C. to 450° C., preferably 250° C. to 350° C. Forexample, the second heat treatment can be performed at 250° C. for onehour in a nitrogen atmosphere.

Through the above steps, the transistor 450 can be formed.

Further, the insulating layer 418 may be formed over the insulatinglayer 412. The insulating layer 418 is preferably formed using aninorganic insulating material that does not contain impurities such asmoisture, a hydrogen ion, and OH⁻ and blocks entry of these impuritiesfrom the outside; for example, a silicon nitride film, an aluminumnitride film, a silicon nitride oxide film, an aluminum oxynitride film,or the like is used. In this embodiment, a silicon nitride film isformed by an RF sputtering method, for example. Since an RF sputteringmethod has high productivity, it is preferably used as a method fordepositing the insulating layer 418 (see FIG. 15D).

Note that depending on the conditions of the first heat treatment andthe second heat treatment or the material of the oxide semiconductorlayer 406 a, part of the oxide semiconductor layer 406 a may becrystallized so that a microcrystal or a polycrystal is formed in theoxide semiconductor layer 406 a. When the oxide semiconductor layer 406a includes a non-single-crystal region, the transistor can have higherfield-effect mobility and larger on-state current. On the other hand,when the oxide semiconductor layer 406 a is amorphous, variations incharacteristics of a plurality of elements can be reduced.

By performing the above-described first heat treatment, hydrogencontained in the oxide semiconductor layer 406 can be reduced(preferably removed), so that the oxide semiconductor layer 406 can behighly purified so as to contain an impurity other than its maincomponent as little as possible. Thus, defects levels generated due toexcessive hydrogen atoms can be reduced. The hydrogen concentration ofthe oxide semiconductor layer 406 at that time is preferably 5×10¹⁹(atoms/cm³) or less. Moreover, the carrier density of the oxidesemiconductor layer 406 is less than 1×10¹⁴ cm⁻³, preferably less than1×10¹² cm⁻³, further preferably less than 1.45×10¹⁹ cm⁻³. That is, thecarrier concentration of the oxide semiconductor layer 406 is as closeto zero as possible. Furthermore, the band gap is 2 eV or more,preferably 2.5 eV or more, further preferably 3 eV or more.

By using the highly purified oxide semiconductor layer 406 for a channelformation region, the off-state current of the transistor can bereduced. The off-state current flows by generation and recombination ofholes and electrons due to direct recombination or indirectrecombination. Since the oxide semiconductor layer has a wide band gapand high thermal energy is needed for exciting electrons, directrecombination and indirect recombination are not likely to occur. Sincea hole which is a minority carrier is substantially zero in an offstate, direct recombination and indirect recombination are not likely tooccur, and the off-state current can be reduced as much as possible.Thus, the transistor can have excellent properties of small off-statecurrent, large on-state current, and high field-effect mobility.

As described above, the highly purified oxide semiconductor layerfunctions as a path, and carriers are supplied from the source and drainelectrodes. By appropriately selecting the electron affinity (χ) and theFermi level (ideally, the Fermi level identical to the intrinsic Fermilevel) of the oxide semiconductor and the work function of the sourceand drain electrodes, carriers can be injected from the source electrodeand the drain electrode while the carrier density of the oxidesemiconductor layer remains low. Thus, an n-channel transistor and ap-channel transistor can be manufactured as appropriate.

The intrinsic carrier density of a highly purified oxide semiconductoris extremely lower than that of silicon. The intrinsic carrier densityof silicon and an oxide semiconductor can be obtained from approximationformulae of the Fermi-Dirac distribution and the Boltzmann distribution.The intrinsic carrier density n_(i) of silicon is 1.45×10¹⁰ cm⁻³ and theintrinsic carrier density n_(i) of an oxide semiconductor (here, anIn—Ga—Zn—O layer) is 1.2×10⁻⁷ cm⁻³, that is, silicon has an intrinsiccarrier density 10¹⁷ times larger than that of the oxide semiconductor.In other words, it is clear that the intrinsic carrier density of theoxide semiconductor is extremely lower than that of silicon.

In this embodiment, the case where a bottom-gate thin film transistor ismanufactured is described; however, one embodiment of the presentinvention is not limited thereto and a top-gate thin film transistor canbe manufactured.

<Electrical Conduction Mechanism of Transistor Including OxideSemiconductor>

Next, an electrical conduction mechanism of a transistor including anoxide semiconductor will be described with reference to FIG. 23, FIGS.24A and 24B, FIGS. 25A and 25B, and FIG. 26. Note that the followingdescription is based on the assumption of an ideal situation forsimplification and does not entirely reflect a real situation. Inaddition, the following description is only an examination.

FIG. 23 is a cross-sectional view of an inverted staggered transistor(thin film transistor) including an oxide semiconductor. An oxidesemiconductor layer (OS) is provided over a gate electrode layer (GE)with a gate insulating layer (GI) therebetween, and a source electrodelayer (S) and a drain electrode layer (D) are provided thereover.

FIGS. 24A and 24B are energy band diagrams (schematic diagrams) of thecross section along A-A′ in FIG. 23. FIG. 24A illustrates the case wherethe potential difference between the source and the drain is zero (thesource and the drain have the same potential, V_(D)=0 V). FIG. 24Billustrates the case where the potential of the drain is higher thanthat of the source (V_(D)>0).

FIGS. 25A and 25B are energy band diagrams (schematic diagrams) of thecross section along B-B′ in FIG. 23. FIG. 25A illustrates a state wherea positive potential (+V_(G)) is applied to the gate (GE1), that is, anon state where carriers (electrons) flow between the source and thedrain. FIG. 25B illustrates a state where a negative potential (−V_(G))is applied to the gate (GE1), that is, an off state (a state whereminority carriers do not flow).

FIG. 26 illustrates the relation between the vacuum level and the workfunction (φ_(M)) of a metal and the relation between the vacuum leveland the electron affinity (χ) of an oxide semiconductor.

Since electrons in the metal are degenerated, the Fermi level is locatedin the conduction band. On the other hand, a conventional oxidesemiconductor is an n-type semiconductor, and the Fermi level (E_(f)) isdistant from the intrinsic Fermi level (E_(i)) in the middle of the bandgap and is located near the conduction band. It is known that hydrogenin the oxide semiconductor is a donor and might be a factor that causesan oxide semiconductor to be an n-type oxide semiconductor. Further,oxygen vacancy is known as one of the causes to produce an n-type oxidesemiconductor.

In contrast, an oxide semiconductor according to one embodiment of theinvention disclosed herein is an oxide semiconductor that is made to beintrinsic (i-type) or to be close to intrinsic in the following manner:hydrogen, which is the cause to produce an n-type oxide semiconductor,is removed from an oxide semiconductor for high purification so that theoxide semiconductor contains an element (impurity element) other thanthe main component of the oxide semiconductor as little as possible, andoxygen vacancy is eliminated. That is, an oxide semiconductor is made tobe or be close to a highly purified i-type (intrinsic) semiconductor notby addition of an impurity element but by elimination of impurities suchas hydrogen and water and oxygen vacancy as much as possible. Thus, theFermi level (E_(f)) can be comparable with the intrinsic Fermi level(E_(i)).

In the case where the band gap (E_(g)) of an oxide semiconductor is 3.15eV, the electron affinity (χ) is said to be 4.3 eV. The work function oftitanium (Ti) included in the source electrode and the drain electrodeis substantially equal to the electron affinity (χ) of the oxidesemiconductor. In that case, a Schottky barrier for electrons is notformed at an interface between the metal and the oxide semiconductor.

That is, in the case where the work function (φ_(M)) of a metal is equalto the electron affinity (χ) of the oxide semiconductor and the metaland the oxide semiconductor are in contact with each other, an energyband diagram (a schematic diagram) illustrated in FIG. 24A is obtained.

In FIG. 24B, a black dot (●) indicates an electron. When a positivepotential is applied to the drain, the electron crosses over a barrier(h) and is injected into the oxide semiconductor, and flows toward thedrain. The height of the barrier (h) depends on a gate voltage and adrain voltage. When a positive drain voltage is applied, the height ofthe barrier (h) is lower than the height of the barrier in FIG. 24Awhere a voltage is not applied, that is, half the band gap (E_(g)).

At this time, as illustrated in FIG. 25A, the electron travels in thevicinity of the interface between the gate insulating layer and thehighly purified oxide semiconductor (the lowest part of the oxidesemiconductor, which is energetically stable).

As illustrated in FIG. 25B, when a negative potential (a reverse bias)is applied to the gate electrode (GE1), a hole which is a minoritycarrier does not substantially exist, so that the current value is asclose to zero as possible.

For example, even when a thin film transistor has a channel width W of1×10⁴ μm and a channel length of 3 μm, the off-state current can be10⁻¹³ A or less and the subthreshold swing (S value) can be 0.1 V/dec atroom temperature (with a 100-nm-thick gate insulating layer).

As described above, the oxide semiconductor is highly purified so as tocontain impurities that are not main components of the oxidesemiconductor as little as possible, so that the thin film transistorcan operate in a favorable manner. For example, the off-state current atroom temperature can be reduced to approximately 1×10⁻²⁰ A (10 zA(zeptoamperes)) to 1×10⁻¹⁹ A (100 zA).

The above-described oxide semiconductor is a highly purified andelectrically intrinsic (i-type) oxide semiconductor made by thefollowing manner: in order to suppress variations in electricalcharacteristics, impurities which cause variations, such as hydrogen,moisture, a hydroxyl group, and hydride (also referred to as hydrogencompound), are removed intentionally; and oxygen which is a maincomponent of the oxide semiconductor and is reduced in the step ofremoving impurities is supplied.

Therefore, it is preferable that the amount of hydrogen in the oxidesemiconductor be as small as possible, and hydrogen contained in theoxide semiconductor is removed to as close to zero as possible so thatthe hydrogen concentration of the oxide semiconductor is 5×10¹⁹(atoms/cm³) or less. The hydrogen concentration of the oxidesemiconductor may be measured by secondary ion mass spectrometry (SIMS).

The number of carriers in the highly purified oxide semiconductor isextremely small (close to zero), and the carrier density is less than1×10¹² cm⁻³, preferably less than 1.45×10¹⁰ cm³. That is, the carrierdensity of the oxide semiconductor layer is extremely close to zero.Since the number of carriers in the oxide semiconductor layer isextremely small, the off-state current of the thin film transistor canbe reduced. It is preferable that the off-state current be as small aspossible. The amount of current of the thin film transistor per achannel width (W) of 1 μm is 100 aA (i.e., 100 aA/μm) or less,preferably 10 aA (i.e., 10 aA/μm) or less, and further preferably 1 aA(i.e., 1 aA/μm) or less. Moreover, since the thin film transistor doesnot have pn junction and hot carrier degradation does not occur,electrical characteristics of the thin film transistor are not adverselyaffected.

As described above, the off-state current can be extremely small in athin film transistor in which an oxide semiconductor layer that ishighly purified by drastically removing hydrogen contained therein isused in a channel formation region. In other words, in circuit design,the oxide semiconductor layer can be regarded as an insulator when thethin film transistor is off. On the other hand, when the thin filmtransistor is on, the current supply capability of the oxidesemiconductor is expected to be higher than that of a semiconductorlayer formed of amorphous silicon.

Design is performed assuming that the off-state current of a thin filmtransistor formed using low-temperature polysilicon is about 10000 timesas large as that of a thin film transistor formed using an oxidesemiconductor. Thus, a period for holding voltage of the thin filmtransistor formed using an oxide semiconductor can be about 10000 timesas long as that of the thin film transistor formed using low-temperaturepolysilicon, when these thin film transistors have an equivalent storagecapacitance (of about 0.1 pF). For example, when moving images aredisplayed at 60 frames per second, a holding period for one signalwriting can be approximately 160 seconds, which is 10000 times as longas that of the thin film transistor formed using low-temperaturepolysilicon. In this manner, still images can be displayed on a displayportion even by less frequent writing of image signals.

By application of the transistor in this embodiment to any of thesemiconductor devices in Embodiments 1 to 3, the drive capability of thesemiconductor device can be improved.

Embodiment 6

In this embodiment, an example of a display device will be described.

FIG. 18A illustrates an example of a display device. The display devicein FIG. 18A includes a circuit 5361, a circuit 5362, a circuit 5363_1, acircuit 53632, and a pixel portion 5364. A plurality of wirings 5371that are extended from the circuit 5362 and a plurality of wirings 5372that are extended from the circuits 5363_1 and 5363_2 are provided inthe pixel portion 5364. In addition, pixels 5367 are arranged in matrixin respective regions where the plurality of wirings 5371 and theplurality of wirings 5372 intersect with each other.

The circuit 5361 has a function of controlling the timing at which thecircuit 5362, the circuit 5363_1, and the circuit 5363_2 operate.Accordingly, the circuit 5361 supplies a signal, voltage, current, orthe like to the circuits 5362, 5363_1, and 53632 in response to a videosignal 5360. For example, the circuit 5361 supplies a source driverstart signal (SSP), a source driver clock signal (SCK), a source driverinverted clock signal (SCKB), video signal data (DATA), and a latchsignal (LAT) to the circuit 5362. Moreover, the circuit 5361 supplies agate driver start signal (GSP), a gate driver clock signal (GCK), and agate driver inverted clock signal (GCKB) to the circuit 5363_1 and thecircuit 5363_2. As described above, the circuit 5361 has a function of acontroller, a control circuit, a timing generator, a power supplycircuit, a regulator, or the like.

The circuit 5362 has a function of outputting video signals to theplurality of wirings 5371 in response to a signal supplied from thecircuit 5361 (e.g., SSP, SCK, SCKB, DATA, or LAT). That is, the circuit5362 has a function of a source driver.

The circuit 5363_1 and the circuit 5363_2 each have a function ofoutputting gate signals to the plurality of wirings 5372 in response toa signal supplied from the circuit 5361 (e.g., GSP, GCK, or GCKB). Thatis, each of the circuit 5363_1 and the circuit 5363 _(—)2 can functionas a gate driver.

Note that in the display device in FIG. 18A, the same signal is suppliedto the circuit 5363_1 and the circuit 53632, so that the circuit 5363_1and the circuit 53632 often output gate signals to the plurality ofwirings 5372 at about the same timing Therefore, the load of the circuit5363_1 and the circuit 53632 can be reduced. However, one example ofthis embodiment is not limited to this structure. For example, asillustrated in FIG. 18B, different signals can be input to the circuit5363_1 and the circuit 5363_2. Accordingly, part of the plurality ofwirings 5372 (e.g., odd-numbered rows) can be controlled by the circuit5363_1 and another part of the plurality of wirings 5372 (e.g.,even-numbered rows) can be controlled by the circuit 5363_2. Thus, thedrive frequency of the circuits 5363_1 and 5363_2 can be lowered.

As illustrated in FIG. 18B, the display device can include a circuit5365 and a lighting device 5366. The circuit 5365 has a function ofcontrolling the amount of electric power supplied to the lighting device5366, time to supply the electric power to the lighting device 5366, orthe like in response to a backlight control signal (BLC) supplied fromthe circuit 5361. Thus, the luminance (or average luminance) of thelighting device 5366 can be controlled in accordance with the videosignal 5360, so that local dimming can be realized. The luminance of thelighting device 5366 can be reduced when an image as a whole is dark,whereas the luminance of the lighting device 5366 can be increased whenan image as a whole is bright. In this manner, the contrast ratio can beincreased or power consumption can be reduced.

The plurality of wirings 5371 and the plurality of wirings 5372 functionas signal lines. Specifically, the plurality of wirings 5371 function assource signal lines (also referred to as video signal lines), and theplurality of wirings 5372 function as gate signal lines (also referredto as scan signal lines or selection signal lines).

Note that one of the circuit 5363_1 and the circuit 5363_2 can beomitted. Alternatively, a circuit having a function similar to that ofthe circuits 5363_1 and 5363_2 can be additionally provided.

Note that one or a plurality of wirings (e.g., a capacitor line, a powersupply line, a gate signal line, and/or a source signal line) can beprovided in the pixel portion 5364 depending on the structure of thepixel 5367. In such a case, it is possible to additionally provide acircuit for controlling a potential of the wiring additionally provided.Specifically, when a liquid crystal element, an electrophoretic element,or the like is used as a display element, a capacitor line is preferablyprovided in the pixel portion 5364. Specifically, when an EL element isused as a display element, a power source is preferably provided in thepixel portion 5364.

As illustrated in FIG. 19A, in the display device of FIG. 18A, thecircuit 5362, the circuit 5363_1, and the circuit 5363_2 can be formedover the substrate 5380 where the pixel portion 5364 is formed.Moreover, the circuit 5361 in the display device of FIG. 18A can beformed over a substrate different from the substrate where the pixelportion 5364 is formed.

As illustrated in FIG. 19B, in the display device of FIG. 18A, thecircuit 5361 and the circuit 5362 can be formed over a substratedifferent from the substrate where the pixel portion 5364 is formed.Since the drive frequency of the circuit 5363_1 and the circuit 5363_2is often lower than that of the circuit 5361 and the circuit 5362, thecircuit 5361 and the circuit 5362 are preferably formed over a substratedifferent from the substrate where the circuit 5363_1 and the circuit53632 are formed. Thus, the drive frequency of the circuit 5361 and thecircuit 5362 can be increased, so that the size of the display devicecan be increased. Furthermore, the circuit 5363_1 and the circuit 53632can be formed over the substrate where the pixel portion 5364 is formed,so that the display device can be manufactured at lower cost.

As illustrated in FIG. 19C, in the display device of FIG. 18A, a circuit5362 a (part of the circuit 5362) can be formed over the substrate wherethe pixel portion 5364 is formed, and the circuit 5361 and a circuit5362 b (another part of the circuit 5362) can be formed over a substratedifferent from the substrate where the pixel portion 5364 is formed. Acircuit whose drive frequency is comparatively low, such as a switch, ashift register, and/or a selector can be used as the circuit 5362 a.Thus, the drive frequency of the circuit 5361 and the circuit 5362 b canbe increased, so that the size of the display device can be increased.Alternatively, the circuit 5362 a, the circuit 5363_1, and the circuit5363_2 can be formed over the substrate where the pixel portion 5364 isformed, so that the display device can be manufactured at lower cost.

As illustrated in FIG. 19D, in the display device in FIG. 18A, a circuit5361 a (part of the circuit 5361) can be formed over the substrate wherethe pixel portion 5364 is formed, and a circuit 5361 b (another part ofthe circuit 5361) can be formed over a substrate different from thesubstrate where the pixel portion 5364 is formed.

A circuit formed over a substrate different from the substrate where thepixel portion 5364 is formed (such a circuit is also referred to as anexternal circuit) often supplies a signal, voltage, current, or the likethrough an input terminal 5381 to a circuit or a wiring formed over thesubstrate where the pixel portion 5364 is formed.

Note that the external circuit can be mounted on an FPC (flexibleprinted circuit) by TAB (tape automated bonding). Alternatively, theexternal circuit can be mounted on the substrate 5380 where the pixelportion 5364 is formed, by COG (chip on glass).

The external circuit is preferably formed using a single crystalsubstrate, an SOI substrate, or the like. Accordingly, it is possible torealize improvement in drive frequency, improvement in drive voltage,reduction in variation of output signals, or the like.

Note that any of the semiconductor devices shown in Embodiments 1 to 4can be applied to the display device in this embodiment. Specifically,any of the semiconductor devices in Embodiments 1 to 4 can be used asthe circuit 5362 and the circuit 5363. Thus, the drive capability of acircuit for driving the pixel portion 5364 (e.g., the circuit 5362 andthe circuit 5363) can be improved, so that the resolution of the pixelcan be increased or the size of the display device can be increased.

Note that in this specification and the like, a display element, adisplay device which is a device including a display element, alight-emitting element, and a light-emitting device which is a deviceincluding a light-emitting element can employ various modes or caninclude various elements. For example, a display medium whose contrast,luminance, reflectance, transmittance, or the like changes byelectromagnetic action, such as an EL (electroluminescence) element(e.g., an EL element including organic and inorganic materials, anorganic EL element, or an inorganic EL element), an LED (e.g., a whiteLED, a red LED, a green LED, or a blue LED), a transistor (a transistorthat emits light depending on the amount of current), an electronemitter, a liquid crystal element, electronic ink, an electrophoreticelement, a grating light valve (GLV), a plasma display panel (PDP), adigital micromirror device (DMD), or a piezoelectric ceramic display,can be used for a display element, a display device, a light-emittingelement, or a light-emitting device. An example of display devicesincluding EL elements is an EL display. Examples of display devicesincluding electron emitters are a field emission display (FED) and anSED-type flat panel display (SED: surface-conduction electron-emitterdisplay). Examples of display devices including liquid crystal elementsare liquid crystal displays (e.g., a transmissive liquid crystaldisplay, a transflective liquid crystal display, a reflective liquidcrystal display, a direct-view liquid crystal display, and a projectionliquid crystal display). An example of display devices includingelectronic ink or electrophoretic elements is electronic paper.

An example of an EL element is an element including an anode, a cathode,and an EL layer placed between the anode and the cathode. Examples of anEL layer are a layer utilizing light emission (fluorescence) from asinglet exciton, a layer utilizing light emission (phosphorescence) froma triplet exciton, a layer utilizing light emission (fluorescence) froma singlet exciton and light emission (phosphorescence) from a tripletexciton, a layer formed using an organic material, a layer formed usingan inorganic material, a layer formed using an organic material and aninorganic material, a layer including a high-molecular material, a layerincluding a low-molecular material, and a layer including ahigh-molecular material and a low-molecular material. Note that varioustypes of EL elements can be used without limitation to the above.

An example of liquid crystal elements is an element that controlstransmission and non-transmission of light by optical modulation actionof liquid crystals. The element can include a pair of electrodes and aliquid crystal layer. The optical modulation action of liquid crystalsis controlled by an electric field applied to the liquid crystal(including a lateral electric field, a vertical electric field, and adiagonal electric field). Specifically, the following can be used for aliquid crystal element, for example: a nematic liquid crystal, acholesteric liquid crystal, a smectic liquid crystal, a discotic liquidcrystal, a thermotropic liquid crystal, a lyotropic liquid crystal, alow-molecular liquid crystal, a high-molecular liquid crystal, a polymerdispersed liquid crystal (PDLC), a ferroelectric liquid crystal, ananti-ferroelectric liquid crystal, a main-chain liquid crystal, aside-chain high-molecular liquid crystal, a plasma addressed liquidcrystal (PALC), and a banana-shaped liquid crystal. Moreover, thefollowing methods can be used for driving the liquid crystals, forexample: a TN (twisted nematic) mode, an STN (super twisted nematic)mode, an IPS (in-plane-switching) mode, an FFS (fringe field switching)mode, an MVA (multi-domain vertical alignment) mode, a PVA (patternedvertical alignment) mode, an ASV (advanced super view) mode, an ASM(axially symmetric aligned microcell) mode, an OCB (opticallycompensated birefringence) mode, an ECB (electrically controlledbirefringence) mode, an FLC (ferroelectric liquid crystal) mode, an AFLC(anti-ferroelectric liquid crystal) mode, a PDLC (polymer dispersedliquid crystal) mode, a PNLC (polymer network liquid crystal) mode, aguest-host mode, and a blue phase mode. Note that various liquid crystalelements and driving methods can be used without limitation to theabove.

For example, display of electronic paper can be performed usingmolecules (a method utilizing optical anisotropy, dye molecularorientation, or the like), particles (a method utilizingelectrophoresis, particle movement, particle rotation, phase change, orthe like), movement of one end of a film, coloring properties or phasechange of molecules, optical absorption by molecules, or self-lightemission by combination of electrons and holes. Specific examples ofdisplay methods of electronic paper are microcapsule electrophoresis,horizontal electrophoresis, vertical electrophoresis, a sphericaltwisting ball, a magnetic twisting ball, a columnar twisting ball, acharged toner, electro liquid powder (registered trademark),magnetophoresis, a magnetic thermosensitive type, electrowetting,light-scattering (transparent-opaque change), a cholesteric liquidcrystal and a photoconductive layer, a cholesteric liquid crystal, abistable nematic liquid crystal, a ferroelectric liquid crystal, aliquid crystal dispersed type with a dichroic dye, a movable film,coloring and decoloring properties of a leuco dye, photochromism,electrochromism, electrodeposition, and flexible organic EL. Note thatvarious electronic paper and display methods can be used withoutlimitation to the above. By using microcapsule electrophoresis as adisplay method of electronic paper, problems of electrophoresis, thatis, aggregation and precipitation of electrophoretic particles can besolved. By using electro liquid powder as a display method of electronicpaper, the electronic paper has advantages such as high-speed response,high reflectance, wide viewing angle, low power consumption, and memoryproperties.

Note that electroluminescence, a cold cathode fluorescent lamp, a hotcathode fluorescent lamp, an LED, a laser light source, a mercury lamp,or the like can be used as a light source of a display device in which alight source is needed, such as a liquid crystal display (e.g., atransmissive liquid crystal display, a transflective liquid crystaldisplay, a reflective liquid crystal display, a direct-view liquidcrystal display, or a projection liquid crystal display), a displaydevice including a grating light valve (GLV), or a display deviceincluding a digital micromirror device (DMD), for example. Note that avariety of light sources can be used without limitation to the above.

Note that in this specification and the like, a transistor can be formedusing a variety of substrates. There is no particular limitation on thekind of a substrate. Examples of a substrate where a transistor isformed are a semiconductor substrate (e.g., a single crystal substrateand a silicon substrate), an SOI substrate, a glass substrate, a quartzsubstrate, a plastic substrate, a metal substrate, a stainless steelsubstrate, a substrate including stainless steel foil, a tungstensubstrate, a substrate including tungsten foil, a flexible substrate, anattachment film, paper including a fibrous material, and a base materialfilm. Examples of a glass substrate are a barium borosilicate glasssubstrate, an aluminoborosilicate glass substrate, and a soda lime glasssubstrate. For a flexible substrate, a flexible synthetic resin such asplastics typified by polyethylene terephthalate (PET), polyethylenenaphthalate (PEN), and polyether sulfone (PES), or acrylic can be used,for example. An example of an attachment film is an attachment filmformed using polypropylene, polyester, vinyl, polyvinyl fluoride,polyvinyl chloride, or the like. For a base film, polyester, polyamide,polyimide, an inorganic vapor deposition film, paper, or the like can beused, for example. Specifically, when a transistor is formed using asemiconductor substrate, a single crystal substrate, an SOI substrate,or the like, it is possible to form a transistor with few variations incharacteristics, size, shape, or the like and with high current supplycapability and a small size. By forming a circuit using suchtransistors, power consumption of the circuit can be reduced or thecircuit can be highly integrated.

Note that a transistor may be formed using one substrate, and then, thetransistor may be transferred to another substrate. Example of asubstrate to which a transistor is transferred are, in addition to theabove-described substrate where the transistor can be formed, a papersubstrate, a cellophane substrate, a stone substrate, a wood substrate,a cloth substrate (including a natural fiber (e.g., silk, cotton, orhemp), a synthetic fiber (e.g., nylon, polyurethane, or polyester), aregenerated fiber (e.g., acetate, cupra, rayon, or regeneratedpolyester), or the like), a leather substrate, and a rubber substrate.By using such a substrate, transistors with excellent properties ortransistors with low power consumption can be formed, a device with highdurability or high heat resistance can be formed, or reduction in weightor thickness can be achieved.

Note that all the circuits that are necessary to realize a desiredfunction can be formed using one substrate (e.g., a glass substrate, aplastic substrate, a single crystal substrate, or an SOI substrate).

In addition, not all the circuits that are necessary to realize thepredetermined function are needed to be formed using one substrate. Thatis, part of the circuits which are necessary to realize thepredetermined function can be formed using one substrate and anotherpart of the circuits which are necessary to realize the predeterminedfunction can be formed using another substrate. For example, some of thecircuits which are necessary to realize the predetermined function canbe formed using a glass substrate and some of the circuits which arenecessary to realize the predetermined function can be formed using asingle crystal substrate (or an SOI substrate). Then, the single crystalsubstrate where some of the circuits which are necessary to realize thepredetermined function (such a substrate is also referred to as an ICchip) are formed can be connected to the glass substrate by COG (chip onglass), and the IC chip can be provided over the glass substrate.Alternatively, the IC chip can be connected to the glass substrate byTAB (tape automated bonding), COF (chip on film), SMT (surface mounttechnology), a printed circuit board, or the like.

Note that the transistor in Embodiment 5 can be used as a transistorincluded in a driver circuit (e.g., the circuit 5362 and the circuit5363) and/or a transistor included in the pixel portion 5354.

Embodiment 7

In this embodiment, an example of a pixel and a method for driving thepixel will be described. Specifically, an example of a pixel thatincludes a display element with memory properties and an example of amethod for driving the pixel will be described.

FIG. 20A illustrates an example of a circuit diagram of a pixel. A pixel5450 includes a transistor 5451, a capacitor 5452, and a display element5453. A first terminal of the transistor 5451 is connected to a wiring5461. A second terminal of the transistor 5451 is connected to oneelectrode of the capacitor 5452 and one electrode of the display element5453 (also referred to as a pixel electrode). A gate of the transistor5451 is connected to a wiring 5462. The other electrode of the capacitor5452 is connected to a wiring 5463. The other electrode of the displayelement 5453 is connected to an electrode 5454 (also referred to as acommon electrode, a counter electrode, or a cathode electrode).

Note that an electrode 5455 refers to one electrode of the displayelement 5453.

The display element 5453 preferably has memory properties. Examples ofthe display element 5453 and a method for driving the display elementare microcapsule electrophoresis, microcup electrophoresis, horizontalelectrophoresis, vertical electrophoresis, twisting ball, liquid powderdisplay, electro liquid powder, a cholesteric liquid crystal, a chiralnematic liquid crystal, an anti-ferroelectric liquid crystal, a polymerdispersed liquid crystal, a charged toner, electrowetting,electrochromism, and electrodeposition.

FIG. 20B is a cross-sectional view of a pixel using microcapsuleelectrophoresis. A plurality of microcapsules 5480 are placed between anelectrode 5454 and an electrode 5455. The plurality of microcapsules5480 are fixed by a resin 5481. The resin 5481 functions as a binder.The resin 5481 preferably has light-transmitting properties. A spaceformed by the electrode 5454, the electrode 5455, and the microcapsule5480 can be filled with a gas such as air or an inert gas. Note that themicrocapsules 5480 can be fixed by formation of a layer including anadhesive or the like on a surface of one or both of the electrodes 5454and 5455.

The microcapsule 5480 includes a film 5482, a liquid 5483, particles5484, and particles 5485. The liquid 5483, the particles 5484, and theparticles 5485 are sealed in the film 5482. The film 5482 haslight-transmitting properties. The liquid 5483 functions as a dispersionliquid. The particles 5484 and the particles 5485 can be dispersed inthe film 5482 by the liquid 5483. It is preferable that the liquid 5483have light-transmitting properties and be not colored. The particle 5484and the particle 5485 have different colors. For example, it ispreferable that one of the particle 5484 and the particle 5485 be blackand the other of the particle 5484 and the particle 5485 be white. Notethat the particle 5484 and the particle 5485 are charged so that theirelectric charge densities are different from each other. For example,one of the particle 5484 and the particle 5485 is positively charged andthe other of the particle 5484 and the particle 5485 is negativelycharged. Thus, when a potential difference occurs between the electrode5454 and the electrode 5455, the particle 5484 and the particle 5485move in accordance with the direction of electric fields. Accordingly,the reflectance of the display element 5453 is changed, so that the graylevel can be controlled. Note that the structure of the microcapsule5480 is not limited to the above-described structure. For example, theliquid 5483 can be colored. As another example, as particles sealed inthe film 5482, one kind of particles or three kinds or more of particlescan be used. As another example, colors of the particle 5484 and theparticle 5485 can be selected from red, green, blue, cyan, magenta,yellow emerald green, vermillion, and the like in addition to white andblack.

The film 5482 is formed using a light-transmitting material (e.g., apolymer resin such as an acrylic resin (e.g., poly(methyl methacrylate)or poly(ethyl methacrylate)), a urea resin, or gum arabic), for example.Note that the film 5482 is preferably gelatinous. By using such a film5482, the plasticity, bending strength, mechanical strength, and thelike can be improved, leading to improvement in flexibility.Alternatively, the microcapsules 5480 can be uniformly arranged with nogap therebetween over a substrate such as film.

A light-transmitting oily liquid is preferably used as the liquid 5483.Specific examples of the liquid 5483 are an alcohol-based solvent (e.g.,methanol, ethanol, isopropanol, butanol, octanol, and methylcellosolve), ester (e.g., ethyl acetate and butyl acetate), aliphatichydrocarbon (e.g., ketone such as acetone, methyl ethyl ketone, andmethyl isobutyl ketone; pentane, hexane, and octane), alicyclichydrocarbon (e.g., cyclohexane and methylcyclohexane), aromatichydrocarbon such as benzene having a long-chain alkyl group (e.g.,benzene, toluene, xylene, hexylbenzene, butylbenzene, octylbenzene,nonylbenzene, decylbenzene, undecylbenzene, dodecylbenzene,tridecylbenzene, and tetradecylbenzene), halogenated hydrocarbon (e.g.,methylene chloride, chloroform, carbon tetrachloride, anddichloroethane), calboxylate salt, water, and other kinds of oils. Otherexamples of the liquid 5483 are a mixture of two or more of theabove-described materials, a combination of a surface active agent orthe like and one of the above materials, and a combination of a surfaceactive agent or the like and a mixture of two or more of the abovematerials.

Each of the particle 5484 and the particle 5485 is formed using apigment. The pigments included in the particle 5484 and the particle5485 preferably have different colors. For example, it is preferablethat the particle 5484 be formed using a black pigment and the particle5485 be formed using a white pigment. Examples of the black pigment areaniline black and carbon black. Examples of the white pigment aretitanium dioxide, zinc white (zinc oxide), and antimony trioxide. Notethat it is possible to add, to the above-described pigment, a chargecontrolling agent (e.g., electrolyte, a surface active agent, metallicsoap, resin, rubber, oil, varnish, or a compound), a dispersing agent(e.g., a titanium-based coupling agent, an aluminum-based couplingagent, or a silane-based coupling agent), a lubricant agent, astabilizing agent, or the like.

FIG. 21A is a cross-sectional view of a pixel in the case where atwisting ball display method is used for the display element 5453. Inthe twisting ball display method, the reflectance is changed by rotationof a display element in order to control the gray level. The differencefrom FIG. 20B is that instead of the microcapsule 5480, a twisting ball5486 is placed between the electrode 5454 and the electrode 5455. Thetwisting ball 5486 includes a particle 5487 and a cavity 5488 formedaround the particle 5487. The particle 5487 is a spherical particle inwhich a surface of one hemisphere is colored in a given color and asurface of the other hemisphere is colored in a different color. Here,the particle 5487 has a white hemisphere and a black hemisphere. Notethat there is a difference in electric charge density between the twohemispheres. For that reason, by generating a potential differencebetween the electrode 5454 and the electrode 5455, the particle 5487 canbe rotated in accordance with the direction of electric fields. Thecavity 5488 is filled with a liquid. As the liquid, a liquid similar tothe liquid 5483 can be used. Note that the structure of the twistingball 5486 is not limited to the structure illustrated in FIG. 21A. Forexample, the twisting ball 5486 can be a cylinder, an ellipse, or thelike.

FIG. 21B is a cross-sectional view of a pixel in the case where amicrocup electrophoresis method is used for the display element 5453. Amicrocup array can be formed in the following manner: a microcup 5491that is formed using a UV curable resin or the like and has a pluralityof recessed portions is filled with charged pigment particles 5493dispersed in a dielectric solvent 5492, and sealing is performed with asealing layer 5494. An adhesive layer 5495 is preferably formed betweenthe sealing layer 5494 and the electrode 5455. As the dielectric solvent5492, a colorless solvent can be used or a colored solvent of red, blue,or the like can be used. This embodiment shows the case where one kindof charged pigment particles is used; alternatively, two or more kindsof charged pigment particles may be used. The microcup has a wall bywhich cells are separated, and thus has sufficiently high resistance toshock and pressure. Moreover, since the components of the microcup aretightly sealed, adverse effects due to change in environment can bereduced.

FIG. 21C is a cross-sectional view of a pixel in the case where anelectro liquid powder display method is used for the display element5453. The electro liquid powder has fluidity and is a substance havingproperties of fluid and properties of a particle. In this method, cellsare separated by partitions 5504, and electro liquid powders 5502 andelectro liquid powders 5503 are placed in the cell. As the electroliquid powder 5502 and the electro liquid powder 5503, a white particleand a black particle are preferably used. Note that the kinds of theelectro liquid powders 5502 and 5503 are not limited thereto. Forexample, colored particles of two colors which are not white and blackcan be used as the electro liquid powders 5502 and 5503. As anotherexample, one of the electro liquid powder 5502 and the electro liquidpowder 5503 can be omitted.

A signal is input to the wiring 5461. Specifically, a signal forcontrolling the gray level of the display element 5453 (e.g., a videosignal) is input to the wiring 5461. Accordingly, the wiring 5461functions as a signal line or a source signal line (also referred to asa video signal line or a source line). A signal is input to the wiring5462. Specifically, a signal for controlling a conduction state of thetransistor 5451 (e.g., a gate signal, a scan signal, or a selectionsignal) is input to the wiring 5462. Accordingly, the wiring 5462functions as a signal line or a gate signal line (also referred to as ascan signal line or a gate line). A constant voltage is supplied to thewiring 5463. The wiring 5463 is connected to the capacitor 5452.Accordingly, the wiring 5463 functions as a power supply line or acapacitor line. A constant voltage is supplied to the electrode 5454.The electrode 5454 is often shared with a plurality of pixels or all thepixels. Accordingly, the electrode 5454 functions as a common electrode(also referred to as a counter electrode or a cathode electrode).

Note that the signals or voltages input to the wiring 5461, the wiring5462, the wiring 5463, and the electrode 5454 are not limited to theabove, and various other signals or voltages can be input. For example,a signal can be input to the wiring 5463. Thus, the potential of theelectrode 5455 can be controlled, so that the amplitude voltage of asignal input to the wiring 5461 can be reduced. Thus, the wiring 5463can have a function of a signal line. As another example, by changing avoltage supplied to the electrode 5454, a voltage applied to the displayelement 5453 can be adjusted. Thus, the amplitude voltage of a signalinput to the wiring 5461 can be reduced.

The transistor 5451 has a function of controlling electrical continuitybetween the wiring 5461 and the electrode 5455, a function ofcontrolling the timing of supplying the potential of the wiring 5461 tothe electrode 5455, or a function of controlling the timing of selectingthe pixel 5450. In such a manner, the transistor 5451 has a function ofa switch or a selection transistor. The transistor 5451 is an n-channeltransistor. For that reason, the transistor 5451 is turned on when an Hsignal is input to the wiring 5462, and is turned off when an L signalis input to the wiring 5462. Note that transistor 5451 is not limited toan n-channel transistor and can be a p-channel transistor. In that case,the transistor 5451 is turned on when an L signal is input to the wiring5462, and is turned off when an H signal is input to the wiring 5462.The capacitor 5452 has a function of holding the potential differencebetween the electrode 5455 and the wiring 5463, or a function of keepingthe potential of the electrode 5455 a predetermined value. Thus, avoltage can continue to be applied to the display element 5453 even whenthe transistor 5451 is off. In such a manner, the capacitor 5452 has afunction of a storage capacitor. Note that functions of the transistor5451 and the capacitor 5452 are not limited to the above, and thetransistor 5451 and the capacitor 5452 can have various other functions.

Next, operation of the pixel in this embodiment will be roughlydescribed. The gray level of the display element 5453 is controlled byapplying a voltage to the display element 5453 so that an electric fieldis generated in the display element 5453. A voltage applied to thedisplay element 5453 is controlled by controlling the potential of theelectrode 5454 and the potential of the electrode 5455. Specifically,the potential of the electrode 5454 is controlled by controlling avoltage applied to the electrode 5454. The potential of the electrode5455 is controlled by controlling a signal input to the wiring 5461. Thesignal input to the wiring 5461 is supplied to the electrode 5455 whenthe transistor 5451 is turned on.

Note that the gray level of the display element 5453 can be controlledby controlling the intensity or the direction of electric fields appliedto the display element 5453, the time to apply electric fields to thedisplay element 5453, or the like. Note that the gray level of thedisplay element 5453 can be maintained by not generating a potentialdifference between the electrode 5454 and the electrode 5455.

Next, an example of operation of the pixel in this embodiment will bedescribed. The timing chart in FIG. 22A shows a period T including aselection period and a non-selection period. The period T is a periodfrom the start of a selection period until the start of the nextselection period.

In the selection period, an H signal is input to the wiring 5462, sothat the potential of the wiring 5462 (referred to as a potential V5462)is at the H level. For that reason, the transistor 5451 is turned on, sothat electrical continuity is established between the wiring 5461 andthe electrode 5455. Thus, a signal input to the wiring 5461 is suppliedto the electrode 5455 through the transistor 5451. Then, the potentialof the electrode 5455 (referred to as a potential V5455) becomes equalto the signal input to the wiring 5461. At this time, the capacitor 5452holds a potential difference between the electrode 5455 and the wiring5463. In the non-selection period, an L signal is input to the wiring5462, so that the potential of the wiring 5462 is at the L level. Forthat reason, the transistor 5451 is turned off, so that electricalcontinuity between the wiring 5461 and the electrode 5455 is broken.Then, the electrode 5455 is set in a floating state. At this time, thecapacitor 5452 holds the potential difference in the selection periodbetween the electrode 5455 and the wiring 5463. For that reason, thepotential of the electrode 5455 remains equal to the signal input to thewiring 5461 in the selection period. In such a manner, in thenon-selection period, a voltage can continue to be applied to thedisplay element 5453 even when the transistor 5451 is off. As describedabove, by controlling a signal input to the wiring 5461 in the selectionperiod, a voltage applied to the display element 5453 can be controlled.That is, the gray level of the display element 5453 can be controlled bycontrolling a signal input to the wiring 5461 in the selection period.

Note that the potential of the electrode 5455 in the non-selectionperiod may be different from the signal input to the wiring 5461 in theselection period because of adverse effects of the off-state current ofthe transistor 5451, feedthrough of the transistor 5451, chargeinjection of the transistor 5451, or the like.

As illustrated in FIG. 22B, the potential of the electrode 5455 can beequal to that of the electrode 5454 in part of the selection period. Forthat reason, by changing the potential of the electrode 5455 in part ofthe selection period even when the same signal is continuously input tothe wiring 5461, the intensity of electric fields applied to the displayelement 5453 can be changed. Thus, afterimages can be reduced; theresponse speed can be increased; or variations in response speed betweenpixels can be reduced so that unevenness or afterimages can beprevented. In order to realize such a driving method, the selectionperiod can be divided into a period T1 and a period T2. In the periodT1, the signal input to the wiring 5461 is preferably equal to thepotential of the electrode 5454. In the period T2, the signal input tothe wiring 5461 preferably has various values in order to control thegray level of the display element 5453. Note that when the period T1 istoo long, a period during which a signal for controlling the gray levelof the display element 5453 is written into the pixel 5450 becomesshort. Therefore, the period T1 is preferably shorter than the periodT2. Specifically, the period T1 accounts for preferably 1 to 20%, morepreferably 3 to 15%, further preferably 5 to 10% of the selectionperiod.

Next described is an example of operation of the pixel in thisembodiment, in which the gray level of the display element 5453 iscontrolled by the time to apply a voltage to the display element 5453.The timing chart in FIG. 22C shows a period Ta and a period Tb. Theperiod Ta includes N periods T (N is a natural number). The N periods Tare similar to the period T illustrated in FIG. 22A or FIG. 22B. Theperiod Ta is a period for changing the gray level of the display element5453 (e.g., an address period, a writing period, or an image rewritingperiod). The period Tb is a period for holding the gray level of thedisplay element 5453 in the period Ta (i.e., a holding period).

A voltage V0 is supplied to the electrode 5454, so that a potential V0is applied to the electrode 5454. A signal having at least three valuesis input to the wiring 5463 and three potentials of the signal are apotential VH (VH>V0), the potential V0, and a potential VL (VL<V0);accordingly, the potential VH, the potential V0, and the potential VLare selectively applied to the electrode 5455.

In each of the N periods T in the period Ta, by controlling a potentialapplied to the electrode 5455, a voltage applied to the display element5453 can be controlled. For example, when the potential VH is applied tothe electrode 5455, the potential difference between the electrode 5454and the electrode 5455 becomes VH−VL. Thus, a positive voltage can beapplied to the display element 5453. When the potential V0 is applied tothe electrode 5455, the potential difference between the electrode 5454and the electrode 5455 becomes zero. Thus, zero voltage can be appliedto the display element 5453. When the potential VL is applied to theelectrode 5455, the potential difference between the electrode 5454 andthe electrode 5455 becomes VL−VH. Thus, a negative voltage can beapplied to the display element 5453. As described above, in the periodTa, the positive voltage (VH−VL), the negative voltage (VL−VH), and zerovoltage can be applied to the display element 5453 in a variety oforders. Thus, the gray level of the display element 5453 can be closelycontrolled; afterimages can be reduced; or the response speed can beincreased.

Note that in this embodiment, when a positive voltage is applied to thedisplay element 5453, the gray level of the display element 5453 isclose to black (also referred to as a first gray level). When a negativevoltage is applied to the display element 5453, the gray level of thedisplay element 5453 is close to white (also referred to as a secondgray level). When zero voltage is applied to the display element 5453,the gray level of the display element 5453 is maintained.

In the period Tb, a signal input to the wiring 5461 is not written intothe pixel 5450. Therefore, a potential applied to the electrode 5455 inthe Nth period T in the period Ta continues to be applied in the periodTb. Specifically, it is preferable that in the period Tb, the gray levelof the display element 5453 be maintained by not generating a fieldeffect in the display element 5453. For that reason, in the Nth period Tin the period Ta, the potential V0 is preferably applied to theelectrode 5455. Thus, the potential V0 is applied to the electrode 5455also in the period Tb, so that zero voltage is applied to the displayelement 5453. In such a manner, the gray level of the display element5453 can be maintained.

Note that as the gray level to be subsequently expressed by the displayelement 5453 is closer to the first gray level, the time during whichthe potential VH is applied to the electrode 5455 is preferably longerin the period Ta. Alternatively, the frequency of application of thepotential VH to the electrode 5455 is preferably higher in the N periodsT. Alternatively, in the period Ta, it is preferable to increase a timeobtained by subtracting the time during which the potential VL isapplied to the electrode 5455 from the time during which the potentialVH is applied to the electrode 5455. Further alternatively, in the Nperiods T, it is preferable to increase a frequency obtained bysubtracting the frequency of application of the potential VL to theelectrode 5455 from the frequency of application of the potential VH tothe electrode 5455.

In addition, as the gray level to be subsequently expressed by thedisplay element 5453 is closer to the second gray level, the time duringwhich the potential VL is applied to the electrode 5455 is preferablylonger in the period Ta. Alternatively, the frequency of application ofthe potential VL to the electrode 5455 is preferably higher in the Nperiods T. Alternatively, in the period Ta, it is preferable to increasea time obtained by subtracting the time during which the potential VH isapplied to the electrode 5455 from the time during which the potentialVL is applied to the electrode 5455. Further alternatively, in the Nperiods T, it is preferable to increase a frequency obtained bysubtracting the frequency of application of the potential VH to theelectrode 5455 from the frequency of application of the potential VL tothe electrode 5455.

In the period Ta, a combination of potentials (the potential VH, thepotential V0, and the potential VL) applied to the electrode 5455 candepend not only on the gray level to be subsequently expressed by thedisplay element 5453, but also on the gray level that has been expressedby the display element 5453. For that reason, if a different gray levelhas been expressed by the display element 5453, a combination ofpotentials applied to the electrode 5455 may vary even when the graylevel to be subsequently expressed by the display element 5453 is thesame.

For example, in the period Ta for expressing the gray level that hasbeen expressed by the display element 5453, the time during which thepotential VL is applied to the electrode 5455 is preferably longer inthe period Ta in any of the following cases: the case where the timeduring which the potential VH is applied to the electrode 5455 islonger; the case where a time obtained by subtracting the time duringwhich the potential VL is applied to the electrode 5455 from the timeduring which the potential VH is applied to the electrode 5455 islonger; the case where the frequency of application of the potential VHto the electrode 5455 is higher in the N periods T; or the case where afrequency obtained by subtracting the frequency of application of thepotential VL to the electrode 5455 from the frequency of application ofthe potential VH to the electrode 5455 is higher in the N periods T.Alternatively, the frequency of application of the potential VL to theelectrode 5455 is preferably higher in the N periods T. Alternatively,in the period Ta, it is preferable to increase a time obtained bysubtracting the time during which the potential VH is applied to theelectrode 5455 from the time during which the potential VL is applied tothe electrode 5455. Further alternatively, in the N periods T, it ispreferable to increase a frequency obtained by subtracting the frequencyof application of the potential VH to the electrode 5455 from thefrequency of application of the potential VL to the electrode 5455. Insuch a manner, afterimages can be reduced.

As another example, in the period Ta for expressing the gray level thathas been expressed by the display element 5453, the time during whichthe potential VH is applied to the electrode 5455 is preferably longerin the period Ta in any of the following cases: the case where the timeduring which the potential VL is applied to the electrode 5455 islonger; the case where a time obtained by subtracting the time duringwhich the potential VH is applied to the electrode 5455 from the timeduring which the potential VL is applied to the electrode 5455 islonger; the case where the frequency of application of the potential VLto the electrode 5455 is higher in the N periods T; or the case where afrequency obtained by subtracting the frequency of application of thepotential VH to the electrode 5455 from the frequency of application ofthe potential VL to the electrode 5455 is higher in the N periods T.Alternatively, the frequency of application of the potential VH to theelectrode 5455 is preferably higher in the N periods T. Alternatively,in the period Ta, it is preferable to increase a time obtained bysubtracting the time during which the potential VL is applied to theelectrode 5455 from the time during which the potential VH is applied tothe electrode 5455. Further alternatively, in the N periods T, it ispreferable to increase a frequency obtained by subtracting the frequencyof application of the potential VL to the electrode 5455 from thefrequency of application of the potential VH to the electrode 5455. Insuch a manner, afterimages can be reduced.

Note that the N periods T have the same length; however, the length ofthe N periods T is not limited thereto and the lengths of at least twoof the N periods T can be different from each other. It is particularlypreferable that the length of the N periods T be weighted. For example,in the case where N=4 and the length of the first period T is denoted bya time h, the length of the second period T is preferably a time h×2,the length of the third period T is preferably a time h×4, and thelength of the fourth period T is preferably a time h×8. When the lengthof the N periods T is weighted in such a manner, the frequency ofselection of the pixel 5450 can be reduced and the time to apply avoltage to the display element 5453 can be closely controlled. Thus,power consumption can be reduced.

Note that the potential VH and the potential VL can be selectivelyapplied to the electrode 5454. In this case, it is preferable that thepotential VH and the potential VL be selectively applied also to theelectrode 5455. For example, in the case where the potential VH isapplied to the electrode 5454, zero voltage is applied to the displayelement 5453 when the potential VH is applied to the electrode 5455,whereas a negative voltage is applied to the display element 5453 whenthe potential VL is applied to the electrode 5455. On the other hand, inthe case where the potential VL is applied to the electrode 5454, apositive voltage is applied to the display element 5453 when thepotential VH is applied to the electrode 5455, whereas zero voltage isapplied to the display element 5453 when the potential VL is applied tothe electrode 5455. In such a manner, the signal input to the wiring5461 can have two values (i.e., the signal can be a digital signal). Forthat reason, it is possible to simplify a circuit that outputs a signalto the wiring 5461.

Note that in the period Tb or part of the period Tb, it is possible notto input a signal to the wiring 5461 and the wiring 5462. That is, thewiring 5461 and the wiring 5462 can be set in a floating state.Moreover, in the period Tb or part of the period Tb, it is possible notto input a signal to the wiring 5463. That is, the wiring 5463 can beset in a floating state. Furthermore, in the period Tb or part of theperiod Tb, it is possible not to supply a voltage to the electrode 5454.That is, the electrode 5454 can be set in a floating state.

The display element with memory properties in this embodiment needs tobe supplied with a voltage higher than that for an ordinary liquidcrystal element (e.g., a TN liquid crystal). The drive voltage can beincreased by using the semiconductor device in any of Embodiments 1 to 4which includes the transistor in Embodiment 5 as a circuit for drivingthe display element with memory properties. This is because thetransistor in Embodiment 5 has withstand voltage higher than that of anamorphous silicon thin film transistor (a-Si TFT), a polycrystallinesilicon thin film transistor (p-Si TFT), or the like.

In addition, it is preferable that the transistor shown in Embodiment 5be used as the transistor 5451 included in the pixel that includes thedisplay element with memory properties, as well as using the transistorin Embodiment 5 in the circuit for driving the display element withmemory properties. Thus, the off-state current of the transistor 5451can be reduced, so that the channel width of the transistor 5451 can bereduced or the area of the capacitor 5452 can be reduced. As a result,the area of the pixel can be reduced. Therefore, when the pixel in thisembodiment is provided in a pixel portion of a display device, theresolution of the display device can be increased. Moreover, the circuitfor driving the display element with memory properties and the pixelportion including the display element with memory properties can beeasily formed over one substrate.

Embodiment 8

In this embodiment, examples of electronic devices will be described.

FIGS. 27A to 27H and FIGS. 28A to 28D each illustrate an electronicdevice. These electronic devices can include a housing 5000, a displayportion 5001, a speaker 5003, an LED lamp 5004, operation keys 5005(including a power switch or an operation switch), a connection terminal5006, a sensor 5007 (a sensor having a function of measuring force,displacement, position, speed, acceleration, angular velocity,rotational frequency, distance, light, liquid, magnetism, temperature,chemical substance, sound, time, hardness, electric field, current,voltage, electric power, radiation, flow rate, humidity, gradient,oscillation, odor, or infrared ray), a microphone 5008, or the like.

FIG. 27A illustrates a personal digital assistant that can include aswitch 5009, an infrared port 5010, and the like in addition to theabove-described components. FIG. 27B illustrates a portable imagereproducing device (e.g., a DVD reproducing device) provided with amemory medium, and the image reproducing device can include a seconddisplay portion 5002, a memory medium reading portion 5011, and the likein addition to the above components. FIG. 27E illustrates a mobiletelevision device that can include an antenna 5014 and the like inaddition to the above components. FIG. 27D illustrates a portable gamemachine that can include the memory medium reading portion 5011 and thelike in addition to the above components. FIG. 27C illustrates aprojector that can include a light source 5033, a projector lens 5034,and the like in addition to the above components. FIG. 27F illustrates aportable game machine that can include the second display portion 5002,the memory medium reading portion 5011, and the like in addition to theabove components. FIG. 27G illustrates a television receiver that caninclude a tuner, an image processing portion, and the like in additionto the above components. FIG. 27H illustrates a portable televisionreceiver that can include a charger 5017 capable of transmitting andreceiving signals and the like in addition to the above components. FIG.28A illustrates a display that can include a support base 5018 and thelike in addition to the above-described components. FIG. 28B illustratesa camera that can include an external connecting port 5019, a shutterbutton 5015, an image receiving portion 5016, and the like in additionto the above components. FIG. 28C illustrates a computer that caninclude a pointing device 5020, the external connecting port 5019, areader/writer 5021, and the like in addition to the above components.FIG. 28D illustrates a mobile phone that can include the antenna 5014, atuner of one-segment (1 seg digital TV broadcasts) partial receptionservice for mobile phones and mobile terminals, and the like in additionto the above components.

The electronic devices illustrated in FIGS. 27A to 27H and FIGS. 28A to28D can have a variety of functions, for example, a function ofdisplaying a variety of information (a still image, a moving image, atext image, and the like) on a display portion; a touch panel function;a function of displaying a calendar, date, time, and the like; afunction of controlling process with a variety of software (programs); awireless communication function; a function of being connected to avariety of computer networks with a wireless communication function; afunction of transmitting and receiving a variety of data with a wirelesscommunication function; and a function of reading program or data storedin a memory medium and displaying the program or data on a displayportion. Further, the electronic device including a plurality of displayportions can have a function of displaying image information mainly onone display portion and displaying text information on another displayportion, a function of displaying a three-dimensional image bydisplaying images where parallax is considered on a plurality of displayportions, or the like. Furthermore, the electronic device including animage receiving portion can have a function of photographing a stillimage, a function of photographing a moving image, a function ofautomatically or manually correcting a photographed image, a function ofstoring a photographed image in a memory medium (an external memorymedium or a memory medium incorporated in the camera), a function ofdisplaying a photographed image on a display portion, or the like. Notethat functions which can be provided for the electronic devicesillustrated in FIGS. 27A to 27H and FIGS. 28A to 28D are not limited tothose described above, and the electronic devices can have a variety offunctions.

Next, applications of a semiconductor device will be described.

FIG. 28E illustrates an example in which a semiconductor device isincorporated in a building structure. FIG. 28E illustrates a housing5022, a display portion 5023, a remote controller 5024 which is anoperation portion, a speaker 5025, and the like. The semiconductordevice is incorporated in the building as a wall-hanging type, so thatthe semiconductor device can be provided without requiring a largespace.

FIG. 28F illustrates another example in which a semiconductor device isincorporated in a building. A display panel 5026 is integrated with aprefabricated bath 5027, so that a person who takes a bath can watch thedisplay panel 5026.

Note that although this embodiment gives the wall and the prefabricatedbath as examples of the building, this embodiment is not limited tothese examples and the semiconductor device can be provided in a varietyof buildings.

Next, examples in which the semiconductor device is incorporated with amoving object will be described.

FIG. 28G illustrates an example in which the semiconductor device isprovided in a car. A display panel 5028 is provided in a body 5029 ofthe car and can display information related to the operation of the caror information input from inside or outside of the car on demand. Notethat a navigation function may be provided.

FIG. 28H illustrates an example in which the semiconductor device isincorporated in a passenger airplane. FIG. 28H illustrates a usagepattern when a display panel 5031 is provided for a ceiling 5030 above aseat of the airplane. The display panel 5031 is integrated with theceiling 5030 through a hinge portion 5032, and a passenger can watch thedisplay panel 5031 by extending and contracting the hinge portion 5032.The display panel 5031 has a function of displaying information whenoperated by the passenger.

Note that although this embodiment gives the body of the car and thebody of the plane as examples of the moving body, this embodiment is notlimited to these examples. The semiconductor device can be provided fora variety of moving bodies such as a two-wheel motor vehicle, afour-wheel vehicle (including a car, bus, and the like), a train(including a monorail, a railway, and the like), and a ship.

Any of the semiconductor devices in Embodiments 1 to 4 is preferablyprovided in the electronic device shown in this embodiment. Inparticular, any of the semiconductor devices in Embodiments 1 to 4 ispreferably provided as a circuit for driving the display portion of theelectronic device. When any of the semiconductor devices in Embodiments1 to 4 is provided as a circuit for driving the display portion of theelectronic device, the area of the driver circuit can be reduced and thesize of the display portion can be increased. Alternatively, theresolution of the display portion can be increased.

Embodiment 9

In this embodiment, another example of the structure and themanufacturing method of the semiconductor device according to Embodiment5 will be described with reference to FIGS. 29A to 29D. In thisembodiment, the difference from Embodiment 5 is described in detail andthe description of Embodiment 5 is employed for similar portions.

First, the gate electrode layer 411 is formed over the substrate 400,and then, the gate insulating layer 402 is formed so as to cover thegate electrode layer 411. After that, a first oxide semiconductor layer404 is formed over the gate insulating layer 402.

The first oxide semiconductor layer 404 can be formed using an oxidesemiconductor material which is a three-component metal oxide and can berepresented by In-M_(X)-Zn_(Y)—O_(z) (Y=0.5 to 5). Here, M denotes oneor plural kinds of elements selected from elements of Group 13, such asgallium (Ga), aluminum (Al), and boron (B). Note that the amount of In,M, Zn, and O contained is not limited and the amount of M can be zero(i.e., X can be 0). In contrast, the amount of In and Zn is not zero.That is, the above expression represents In—Ga—Zn—O and In—Zn—O, forexample.

Like the oxide semiconductor layer 406 in Embodiment 5, the first oxidesemiconductor layer 404 can be formed using a four-component metal oxidesuch as an In—Sn—Ga—Zn—O-based metal oxide; a three-component metaloxide such as an In—Ga—Zn—O-based metal oxide, an In—Sn—Zn—O-based metaloxide, an In—Al—Zn—O based metal oxide, a Sn—Ga—Zn—O-based metal oxide,an Al—Ga—Zn—O-based metal oxide, or a Sn—Al—Zn—O-based metal oxide; atwo-component metal oxide such as an In—Zn—O-based metal oxide, aSn—Zn—O-based metal oxide, an Al—Zn—O-based metal oxide, a Zn—Mg—O-basedmetal oxide, a Sn—Mg—O-based metal oxide, or an In—Mg—O-based metaloxide; an In—O-based metal oxide, a Sn—O-based metal oxide, a Zn—O-basedmetal oxide, or the like.

In this embodiment, the first oxide semiconductor layer 404 is formed bya sputtering method with the use of an In—Ga—Zn—O-based oxidesemiconductor target.

As a target used for forming the In—Ga—Zn—O-based first oxidesemiconductor layer 404 by a sputtering method, a metal oxide targetcontaining zinc oxide as its main component can be used, for example.The target containing In, Ga, and Zn has a composition ratio ofIn:Ga:Zn=1:x:y (x is 0 or larger and y is 0.5 to 5). For example, atarget with a composition ratio of In:Ga:Zn=1:1:1 [atomic ratio] (x=1and y=1; that is, In₂O₃:Ga₂O₃:ZnO=1:1:2 [molar ratio]) may be used. Asthe oxide semiconductor target, it is possible to use a target with acomposition ratio of In:Ga:Zn=1:1:0.5 [atomic ratio], a target with acomposition ratio of In:Ga:Zn=1:1:2 [atomic ratio], or a target with acomposition ratio of In:Ga:Zn=1:0:1 [atomic ratio] (x=0 and y=1). Inthis embodiment, it is preferable to use an oxide semiconductor targetwith which crystals are easily generated, because heat treatment isperformed later in order to intentionally crystallize the first oxidesemiconductor layer 404.

Then, the first heat treatment is performed on the first oxidesemiconductor layer 404, so that a region including at least a surfaceof the first oxide semiconductor layer 404 is crystallized (see FIG.29A). By performing the first heat treatment on the first oxidesemiconductor layer 404, an excess amount of water (including a hydroxylgroup), hydrogen, and the like that are contained in the first oxidesemiconductor layer 404 can be removed. The first heat treatment isperformed at 450° C. to 850° C., preferably 550° C. to 750° C. for 1minute to 24 hours.

In this embodiment, as the first heat treatment, heat treatment isperformed at 700° C. for one hour in a nitrogen atmosphere; afterdehydration or dehydrogenation is performed, the atmosphere is switchedto an oxygen atmosphere so that oxygen is supplied inside the firstoxide semiconductor layer 404.

The first heat treatment in Embodiment 5 can be referred to for otherconditions of the heat treatment; therefore, the detailed description isnot repeated.

The first heat treatment is performed on the first oxide semiconductorlayer 404, so that a non-single-crystal region can be formed in theregion including at least the surface of the first oxide semiconductorlayer 404. The non-single-crystal region formed in the region includingthe surface of the first oxide semiconductor layer 404 is formed bycrystal growth from the surface toward the inside. Thenon-single-crystal region is a plate-like non-single-crystal layer withan average thickness of 2 nm to 10 nm. Moreover, the non-single-crystalregion includes a non-single-crystal layer which has a c-axis alignmentin the direction substantially perpendicular to the surface of the firstoxide semiconductor layer 404. Here, “substantially parallel” means astate within ±10° from a parallel direction. Further, “substantiallyperpendicular” means a state within ±10° from a perpendicular direction.

Next, a second oxide semiconductor layer 405 is formed over the firstoxide semiconductor layer 404 (see FIG. 29B).

Like the first oxide semiconductor layer 404, the second oxidesemiconductor layer 405 can be formed using a four-component metal oxidesuch as an In—Sn—Ga—Zn—O-based metal oxide; a three-component metaloxide such as an In—Ga—Zn—O-based metal oxide, an In—Sn—Zn—O-based metaloxide, an In—Al—Zn—O based metal oxide, a Sn—Ga—Zn—O-based metal oxide,an Al—Ga—Zn—O-based metal oxide, or a Sn—Al—Zn—O-based metal oxide; atwo-component metal oxide such as an In—Zn—O-based metal oxide, aSn—Zn—O-based metal oxide, an Al—Zn—O-based metal oxide, a Zn—Mg—O-basedmetal oxide, a Sn—Mg—O-based metal oxide, or an In—Mg—O-based metaloxide; an In—O-based metal oxide, a Sn—O-based metal oxide, a Zn—O-basedmetal oxide, or the like.

It is preferable that the second oxide semiconductor layer 405 be formedusing a material containing the same main component as the first oxidesemiconductor layer 404, or that the second oxide semiconductor layer405 have the same crystal structure as the first oxide semiconductorlayer 404 and have a lattice constant close to the first oxidesemiconductor layer 404 (the mismatch is 1% or lower). In the case wherea material containing the same main component is used for the secondoxide semiconductor layer 405 and the first oxide semiconductor layer404, the second oxide semiconductor layer 405 is easily crystallizedwhen crystals grow using the non-single-crystal region of the firstoxide semiconductor layer 404 as seeds in the second heat treatmentperformed later. Moreover, the case where a material containing the samemain component is used for these oxide semiconductor layers, favorableelectrical characteristics and interface characteristics such asadhesion between the second oxide semiconductor layer 405 and the firstoxide semiconductor layer 404 can be obtained.

Alternatively, the second oxide semiconductor layer 405 may be formedusing a material containing a main component different from that of amaterial for the first oxide semiconductor layer 404. In the case ofusing a material containing a main component different from that of amaterial for the first oxide semiconductor layer 404, electricalcharacteristics of the layers can be different from each other. Forexample, when the second oxide semiconductor layer 405 is formed using amaterial with high electrical conductivity and the first oxidesemiconductor layer 404 is formed using a material with low electricalconductivity, it is possible to realize a semiconductor device in whichadverse effects of the base interface are reduced. Furthermore, when amaterial that is easily crystallized is used for the first oxidesemiconductor layer 404 to form a favorable seed crystal and then, thesecond oxide semiconductor layer 405 is formed and crystallized, thecrystallinity of the second oxide semiconductor layer 405 can befavorable regardless of easiness of crystallization of the second oxidesemiconductor layer 405.

In this embodiment, the second oxide semiconductor layer 405 is formedby a sputtering method with the use of an In—Ga—Zn—O-based oxidesemiconductor target. The second oxide semiconductor layer 405 may bedeposited in a manner similar to that of the first oxide semiconductorlayer 404. Note that the thickness of the second oxide semiconductorlayer 405 is preferably larger than that of the first oxidesemiconductor layer 404. Moreover, it is preferable to form the secondoxide semiconductor layer 405 so that the sum of the thickness of thefirst and second oxide semiconductor layers 404 and 405 is 3 nm to 50 nmNote that since an appropriate thickness differs depending on an oxidesemiconductor material to be used, application, and the like, thethickness is set in accordance with the material to be used,application, and the like.

Next, the second heat treatment is performed on the second oxidesemiconductor layer 405, so that crystals grow using thenon-single-crystal region of the first oxide semiconductor layer 404 asseeds and the crystallized second oxide semiconductor layer 405 isformed (see FIG. 29C).

By performing the second heat treatment on the second oxidesemiconductor layer 405, crystals can grow in the entire second oxidesemiconductor layer 405 from the non-single-crystal region formed at theinterface between the first oxide semiconductor layer 404 and the secondoxide semiconductor layer 405, and the crystallized second oxidesemiconductor layer 405 can be formed. Furthermore, by performing thesecond heat treatment, the first oxide semiconductor layer 404 can be anon-single-crystal layer with higher degree of crystal orientation.

Note that in the first oxide semiconductor layer 404, a regionoverlapping with unevenness of the gate insulating layer 402 has a grainboundary and thus has a non-single-crystal structure. In the secondoxide semiconductor layer 405, a region that serves as a channelformation region at least has a flat surface. Moreover, the regionserving as the channel formation region in the second oxidesemiconductor layer 405 includes c-axis-oriented non-single crystalslike the first oxide semiconductor layer 404. Note that in the regionoverlapping with the gate electrode layer 411 (i.e., the channelformation region), a difference in height of the surface of the secondoxide semiconductor layer 405 is preferably 1 nm or less, furtherpreferably 0.2 nm or less. The a-axis and the b-axis of the non-singlecrystals deviate in the channel formation region of the second oxidesemiconductor layer 405.

For example, when an In—Ga—Zn—O-based oxide semiconductor material isused for the second oxide semiconductor layer 405, the second oxidesemiconductor layer 405 can include a crystal represented byInGaO₃(ZnO), (m is larger than 0 and is not a natural number), a crystalrepresented by In₂Ga₂ZnO₇ (In:Ga:Zn:O=2:2:1:7), or the like. By thesecond heat treatment, such crystals align so that the c-axis issubstantially perpendicular to the surface of the second oxidesemiconductor layer 405.

Here, the above-described crystal includes any of In, Ga, and Zn and canbe considered to have a layered structure of layers parallel to thea-axis and the b-axis. Specifically, the above crystal has a structurein which a layer that contains In and a layer that does not contain In(i.e., a layer that contains Ga or Zn) are stacked in the c-axisdirection.

In the In—Ga—Zn—O-based oxide semiconductor, the conductivity in thedirection parallel to the a-axis and the b-axis of the layer containingIn is favorable. This is because electrical conduction in theIn—Ga—Zn—O-based oxide semiconductor is mainly controlled by In andbecause a carrier path is formed since the 5 s orbital of one Inoverlaps with the 5 s orbital of adjacent In.

When the first oxide semiconductor layer 404 includes an amorphousregion at the interface with the gate insulating layer 402, the secondheat treatment sometimes makes crystal grow from the crystalline regionformed on the surface of the first oxide semiconductor layer 404 towarda bottom surface of the first oxide semiconductor layer 404 so that theamorphous region is crystallized. Note that the amorphous region is leftin some cases depending on the material included in the gate insulatinglayer 402, the conditions of the heat treatment, or the like.

In the case where the first oxide semiconductor layer 404 and the secondoxide semiconductor layer 405 are formed using an oxide semiconductormaterial with the same main component, when crystal grows upward towardthe surface of the second oxide semiconductor layer 405 as illustratedin FIG. 29C, using the first oxide semiconductor layer 404 as the seedsfor crystal growth, the first oxide semiconductor layer 404 and thesecond oxide semiconductor layer 405 have the same crystal structure.For that reason, although shown by dot lines in FIG. 29C, the interfacebetween the first oxide semiconductor layer 404 and the second oxidesemiconductor layer 405 cannot be recognized and the first and secondoxide semiconductor layers 404 and 405 can be regarded as one layer insome cases.

In such a manner, by performing the second heat treatment, the entiresecond oxide semiconductor layer 405 can be crystallized from thenon-single-crystal region formed at the interface between the secondoxide semiconductor layer 405 and the first oxide semiconductor layer404. Furthermore, by performing the second heat treatment, the firstoxide semiconductor layer 404 can be a non-single-crystal layer withhigher degree of crystal orientation.

The second heat treatment is performed at 450° C. to 850° C., preferably600° C. to 700° C. for 1 minute to 100 hours, preferably 5 hours to 20hours, typically 10 hours.

Also in the second heat treatment, it is preferable that water,hydrogen, and the like be not contained in nitrogen, oxygen, or a raregas such as helium, neon, or argon. Alternatively, it is preferable thatthe purity of nitrogen, oxygen, or the rare gas such as helium, neon, orargon which is introduced into a heat treatment apparatus be 6N orhigher, further preferably 7N or higher. The second heat treatment maybe performed in ultra-dry air with an H₂O concentration of 20 ppm orlower, preferably in ultra-dry air with an H₂O concentration of 1 ppm orlower. By such second heat treatment, water (including a hydroxylgroup), hydrogen, and the like that are contained in the second oxidesemiconductor layer 405 can be removed. Thus, it is possible to form thefirst oxide semiconductor layer 404 and the second oxide semiconductorlayer 405 that are made to be intrinsic or substantially intrinsic bybeing highly purified by reduction in impurities.

In addition, the atmosphere inside a furnace may be switched so that anitrogen atmosphere is used when the temperature is increased in thesecond heat treatment and an oxygen atmosphere is used at the time ofcooling; when the atmosphere is switched to an oxygen atmosphere afterdehydration or dehydrogenation is performed in a nitrogen atmosphere,oxygen can be supplied inside the second oxide semiconductor layer 405.

The heat treatment apparatus in Embodiment 5 can be referred to for theheat treatment apparatus used in the second heat treatment; therefore,the detailed description is not repeated.

The subsequent steps can refer to Embodiment 5 (FIGS. 15B to 15D).

Through the above-described process, the transistor 450 including theoxide semiconductor layer 406 a is completed (see FIG. 29D).

The non-single-crystal region is formed in the oxide semiconductor layer406 a as described above, whereby the mobility of the transistor can beincreased. When the transistor whose mobility is increased in such amanner is applied to a circuit for which high-speed operation isrequired, the drive capability of the circuit can be improved.

By application of the transistor in this embodiment to any of thesemiconductor devices in Embodiments 1 to 3, the drive capability of thesemiconductor device can be improved.

In addition, a combination of the transistor in this embodiment and thetransistor in Embodiment 5 can be applied to any of the semiconductordevices in Embodiments 1 to 4.

This application is based on Japanese Patent Application serial no.2009-282268 filed with Japan Patent Office on Dec. 11, 2009, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A semiconductor device comprising: a firsttransistor; a second transistor; a third transistor; and a fourthtransistor, wherein a first terminal of the first transistor iselectrically connected to a first wiring and a second terminal of thefirst transistor is electrically connected to a second wiring, wherein agate of the second transistor is electrically connected to a thirdwiring, a first terminal of the second transistor is electricallyconnected to the third wiring, and a second terminal of the secondtransistor is electrically connected to a gate of the first transistor,wherein a gate of the third transistor is electrically connected to afourth wiring, a first terminal of the third transistor is electricallyconnected to a fifth wiring, and a second terminal of the thirdtransistor is electrically connected to the second wiring, wherein agate of the fourth transistor is electrically connected to the fourthwiring, a first terminal of the fourth transistor is electricallyconnected to the fifth wiring, and a second terminal of the fourthtransistor is electrically connected to the gate of the firsttransistor, wherein a channel width of the first transistor is largerthan a channel width of the second transistor, and wherein the secondtransistor comprises an oxide semiconductor layer comprising a channelformation region.
 2. The semiconductor device according to claim 1, thesecond transistor comprising: a gate electrode layer on a substrate; aninsulating layer having a layered structure on the gate electrode layerand under and in contact with the oxide semiconductor layer, theinsulating layer comprising silicon, oxygen and nitrogen; a sourceelectrode layer and a drain electrode layer on and in contact with theoxide semiconductor layer, each of the source electrode layer and thedrain electrode layer having a layered structure; and a silicon oxidelayer on and in contact with the source electrode layer and the drainelectrode layer.
 3. The semiconductor device according to claim 1,wherein an off-state current of the second transistor per channel widthof 1 μm is 1 aA or less.
 4. The semiconductor device according to claim1, wherein each of the first transistor, the third transistor and thefourth transistor comprises a separate oxide semiconductor layer.
 5. Thesemiconductor device according to claim 1, wherein the oxidesemiconductor layer includes a non-single-crystal region.
 6. Thesemiconductor device according to claim 1, wherein the oxidesemiconductor layer includes a non-single-crystal region, thenon-single-crystal region having a c-axis alignment in a directionperpendicular to a surface of the oxide semiconductor layer.
 7. Adisplay device comprising the semiconductor device according to claim 1,and further comprising: a substrate; a pixel portion on the substrate; agate driver circuit on the substrate, the gate driver circuit beingconfigured to control the pixel portion; and an input terminal on thesubstrate, wherein the semiconductor device is comprised in the gatedriver circuit.
 8. An electronic device comprising the semiconductordevice according to claim
 1. 9. A semiconductor device comprising: asubstrate; a first transistor; a second transistor; a third transistor;and a fourth transistor, wherein a first terminal of the firsttransistor is electrically connected to a first wiring and a secondterminal of the first transistor is electrically connected to a secondwiring, wherein a gate of the second transistor is electricallyconnected to a third wiring, a first terminal of the second transistoris electrically connected to the third wiring, and a second terminal ofthe second transistor is electrically connected to a gate of the firsttransistor, wherein a gate of the third transistor is electricallyconnected to a fourth wiring, a first terminal of the third transistoris electrically connected to a fifth wiring, and a second terminal ofthe third transistor is electrically connected to the second wiring,wherein a gate of the fourth transistor is electrically connected to thefourth wiring, a first terminal of the fourth transistor is electricallyconnected to the fifth wiring, and a second terminal of the fourthtransistor is electrically connected to the gate of the firsttransistor, wherein a channel width of the first transistor is largerthan a channel width of the second transistor, wherein the secondtransistor comprises: a gate electrode layer on the substrate; aninsulating layer having a layered structure on the gate electrode layer,the insulating layer comprising silicon, oxygen and nitrogen; an oxidesemiconductor layer comprising a channel formation region on and incontact with the insulating layer; a source electrode layer and a drainelectrode layer on and in contact with the oxide semiconductor layer,each of the source electrode layer and the drain electrode layer havinga layered structure; and a silicon oxide layer on and in contact withthe oxide semiconductor layer.
 10. The semiconductor device according toclaim 9, further comprising a silicon nitride layer, wherein the siliconoxide layer is on and in contact with the source electrode layer and thedrain electrode layer, and wherein the silicon nitride layer is on andin contact with the silicon oxide layer.
 11. The semiconductor deviceaccording to claim 9, wherein an off-state current of the secondtransistor per channel width of 1 μm is 1 aA or less.
 12. Thesemiconductor device according to claim 9, wherein each of the firsttransistor, the third transistor and the fourth transistor comprises aseparate oxide semiconductor layer.
 13. The semiconductor deviceaccording to claim 9, wherein the oxide semiconductor layer includes anon-single-crystal region.
 14. The semiconductor device according toclaim 9, wherein the oxide semiconductor layer includes anon-single-crystal region, the non-single-crystal region having a c-axisalignment in a direction perpendicular to a surface of the oxidesemiconductor layer.
 15. A display device comprising the semiconductordevice according to claim 9, and further comprising: a pixel portion onthe substrate; a gate driver circuit on the substrate, the gate drivercircuit being configured to control the pixel portion; and an inputterminal on the substrate, wherein the semiconductor device is comprisedin the gate driver circuit.
 16. An electronic device comprising thesemiconductor device according to claim
 9. 17. A semiconductor devicecomprising: a first transistor; a second transistor; a third transistor;and a fourth transistor, wherein a first terminal of the firsttransistor is electrically connected to a first wiring and a secondterminal of the first transistor is electrically connected to a secondwiring, wherein a gate of the second transistor is electricallyconnected to a third wiring, a first terminal of the second transistoris electrically connected to the third wiring, and a second terminal ofthe second transistor is electrically connected to a gate of the firsttransistor, wherein a gate of the third transistor is electricallyconnected to a fourth wiring, a first terminal of the third transistoris electrically connected to a fifth wiring, and a second terminal ofthe third transistor is electrically connected to the second wiring,wherein a first terminal of the fourth transistor is electricallyconnected to the gate of the first transistor, wherein a channel widthof the first transistor is larger than a channel width of the secondtransistor, and wherein the second transistor comprises an oxidesemiconductor layer comprising a channel formation region.
 18. Thesemiconductor device according to claim 17, the second transistorcomprising: a gate electrode layer on a substrate; an insulating layerhaving a layered structure on the gate electrode layer and under and incontact with the oxide semiconductor layer, the insulating layercomprising silicon, oxygen and nitrogen; a source electrode layer and adrain electrode layer on and in contact with the oxide semiconductorlayer, each of the source electrode layer and the drain electrode layerhaving a layered structure; and a silicon oxide layer on and in contactwith the source electrode layer and the drain electrode layer.
 19. Thesemiconductor device according to claim 17, wherein an off-state currentof the second transistor per channel width of 1 μm is 1 aA or less. 20.The semiconductor device according to claim 17, wherein each of thefirst transistor, the third transistor and the fourth transistorcomprises a separate oxide semiconductor layer.
 21. The semiconductordevice according to claim 17, wherein the oxide semiconductor layerincludes a non-single-crystal region.
 22. The semiconductor deviceaccording to claim 17, wherein the oxide semiconductor layer includes anon-single-crystal region, the non-single-crystal region having a c-axisalignment in a direction perpendicular to a surface of the oxidesemiconductor layer.
 23. A display device comprising the semiconductordevice according to claim 17, and further comprising: a substrate; apixel portion on the substrate; a gate driver circuit on the substrate,the gate driver circuit being configured to control the pixel portion;and an input terminal on the substrate, wherein the semiconductor deviceis comprised in the gate driver circuit.
 24. An electronic devicecomprising the semiconductor device according to claim 17.